#dichroscope

[just-here-with-my-thoughts]

Pleochroism in gem quality zoisite var. tanzanite

Apparently we're doing this...

*my pictures, not my crystal

Ever wondered why the colour of some gemstones seems to shift as you view them from different angles? Perhaps are you were looking at a pleochroic gemstone!

Pleochroism is a body colour effect seen in transparent to translucent, optically anisotropic* crystalline material caused by differential absorption of the polarised light rays following different paths through the gemstone, and causing a change to perceived body colour when the stone is viewed from different directions.

...Okay, but what does that mean?

Pleochroism means 'many coloured'. When light passes through a crystal, it is sometimes split into two rays - each ray takes a slightly different path, and because it encounters different atoms within the crystal structure, it is modified to a different colour.

When both different coloured rays of light exit the crystal, your eye resolves them as a single colour. Clever, right? :D

The four photos above are of the same crystal. The top photos show the crystal from two different angles - one looks distinctly more purplish than the other, right? That is eye-visible pleochroism!

Even gems that show pleochroism don't always show it strongly enough to see with the naked eye. But this tanzanite crystal does.

The bottom photos show the same crystals viewed through a type of polarising filter called a dichroscope. The dichroscope is made of polarising film where each half lets through only one of the rays of coloured light - meaning you can see each of the two colours at the same time, one in each half of the filter!

But wait!- I hear you cry. Why are there THREE different colours??

Hmm well this is getting into a deeper discussion of crystal habits and their optical behaviours, so for now let's say that depending on the symmetry of the crystal, an optically anisotropic* stone can show two or three colours - but only ever two in one direction! You have to turn the crystal to see the third colour.

If a stone shows two colours we say it is dichroic, if it shows three it is trichroic. This is the maximum number of colours you will see.

When you view the tanzanite crystal in the position in the left photo, the light rays being transmitted are blue and reddish-purple. Your eyes resolve this to show a purpley-blue colour. Neat!

But in the position from the right photo, it's transmitting blue and yellow. Wild! The yellow ray dilutes some of the intensity of the blue, so the colour you see is a weaker blue tone.

The photo I'm missing from my set is the crystal viewed from top-down, which would then show a purple and yellow split through the dichroscope!

So turning the stone, or moving your head position relative to the position of the stone, really does change its colour, because you are perceiving a different set of coloured light rays being transmitted through the stone!

When fashioning a rough crystal, a lapidary (that's a stone cutter) would to orient the crystal so the best colour is face-up - ie. when you look at the finished polished stone, you will see the most attractive balance of colour. What is considered ideal varies with the stone - you might cut a very dark crystal to show its lighter colour, or a pale one to show the most intense colour it is able to!

Fun additional fact - tanzanite can be heated to improve its colour. It does this by removing the yellow element entirely, meaning you will see intense bluey-purple tones from all angles! The wonders of stone treatments!

*Edit I am misleading you by saying you will see bluey-purple in all directions... in two directions (as tanzanite is a biaxial gemstone) you will find an optic axis, which is a direction of optical isotropy in an otherwise optically anisotropic* gemstone. So in this direction, you will only have a single transmitted ray of light, not two!

*I use the phrase optically anisotropic a couple of times and it's hard to explain without a whole separate citizen lecture on crystallography, but the quick summary is that it means a crystal in which light behaves differently as it travels in different directions - such as the differential absorption resulting in different colours in pleochroism! This is as opposed to an optically isotropic crystal, where light behaves the same way in all directions of travel.

And that is a recited-from-memory summary of pleochroism! :D (please don't tell me off if I got any of it wrong I didn't go upstairs to check my notes ;_;) (I mean I didn't get any of it wrong but my quoted definition probably wasn't word perfect ^^;)

@royallykt thank you so much for your interest i hope you enjoyed learning about all this :)

[perfectgroupindiasblog]

Polariscope | Perfect Group India

Polariscope is a gemological tool that determines if a gemstone is single or double-refractive and enables us to identify the different crystal axes of the stone. That's how it acquired its name: A polariscope is a scope used to view the stone's poles or axis. Using plane polarised light, we can see the many hues of light that a gemstone transmits, if you recall the section on the dichroscope. However, we can really see the path those beams are traveling through the stone thanks to the polariscope. Being aware of the stone's double refractive property allows us to utilize the optic interference figure to determine the different optical directions in which light is passing through the stone.

[gemic-laboratory]

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[evilvillain123456789]

[looking at a jpeg of an adult childrens cartoon character blown up on my ipad through a dichroscope, concentrating, brow furrowed]

[looks up, places everything down then scowls at you] the bellybutton lint on this woman is atrocious, you wanted how much for it? i don't have time for jokes. get out of my sight

[dlvjewel]

Et sous le dichroscope, un pléochroïsme à se damner. https://www.instagram.com/p/COn2QagBQ5A/?igshid=azm185bce0km

[scriptchemist]

Hi! I'm trying to work out ways to differentiate between true and false jade. It's my understanding that though both nephrite (Ca2(Mg, Fe)5Si8O22(OH)2) and serpentine ((Mg, Fe)3Si2O5(OH)4) contain magnesium, nephrite has a higher temperature required in order to burn. So, would it be plausible to test the difference between the two by trying to burn small chips of the sample? Would there be any harmful byproducts? Is there an easier way to tell which is which? Thank you so much!!

The field you’re thinking of is gemology, which is the study of gems and gemstone materials. Of the techniques available to gemologists, combustion isn’t the first, or even the fifth, test they’d reach for. Here are a few reasons why:

Non-destructive methods are preferred. If I send in a piece of jade for analysis, I would expect to get it back whole and hale, not charred, and not have bits of it hacked off. That said, non-destructive isn’t always possible (more on this later), but if we have to go destructive, there are much better ways to do it than fire.

Gemstones and minerals, being natural, are products of their environment and their properties may differ a little from sample to sample. One ruby from one end of the world will likely differ from a ruby obtained from another part of the world, even if they’re both rubies. This is why many properties, such as hardness, specific gravity, etc, are often given as a range. So unless nephrite and serpentine burn at very different temperatures (side note: I have no idea what temperatures they’d burn at) and do not overlap with the burning temperatures of any other precious stone, it’s very hard to definitively say what this sample is based on the results from one test.

I’m answering this under the assumption that modern techniques are available. If modern techniques aren’t available that changes things, but then I assume then your character wouldn’t have the means to measure temperatures precisely at the temperatures required to burn gems and precious stones. 

Also, identifying a sample is a process of elimination–these are not defining characteristics. In other words, these tests tell you what your sample isn’t or what it could be, not what it is, and your character must tick off the possibilities one by one until they figure out what it must be. Your character is trying to determine the identity of this sample out of all the green-coloured hunks of precious stone and minerals possible, not just choosing between nephrite and serpentine (both of which have various sub-types), so it wouldn’t be a binary choice of “oh, it’s B because it’s not A.”

Having said all that, here’s some tests your gemologist character can try to identify their unknown sample (disclaimer: I am not a gemologist):

Observation of colour, lustre, and other visual characteristics

Give your sample a good hard look with the eyes, with a 10x loupe, and/or if your character has access to a full gemologist lab, with a binocular microscope. What’s its colour? What’s its lustre? How’s it cut? If this is a jewellery piece rather than a rough sample, has the surface been oiled or otherwise treated? Do you see fibres running throughout, or crystalline masses? Is there evidence of heat treatment? Is it a composite stone? (Composites are stones that have been mounted on or sandwiched between cheaper materials to lower costs, increase strength, etc. A doublet is a precious stone mounted onto a cheaper backing; a triplet is a precious stone sandwiched between two layers of cheaper material.)

Nephrite’s lustre ranges from vitreous (glassy) to greasy, while serpentine ranges from vitreous, greasy, and silky.  Colours of both vary.

Density (or specific gravity)

Density is defined as mass divided by volume, and specific gravity is a ratio of a sample’s density vs that of water. Specific gravity is very easy to measure: simply weigh your sample in air on a scale, then weigh it again when submerged in water; SG = (mass in air)/(mass in air - mass when submerged).

Nephrite’s specific gravity is approximately 2.9-3.04; serpentine’s is about 2.4-2.8.

Refractive index

Light slows and bends as it enters a gemstone due to its greater density. By measuring how much the light bends as it passes through a material we determine the refractive index of a material, which is a good identifying (but not defining) characteristic. This can be measured with a handheld instrument called the refractometer.

Nephrite’s refractive index is about 1.600 to 1.640; serpentine’s is about 1.490-1.575.

Birefringence

Most precious stones are doubly refractive. When a beam of light enters the stone, it splits into two beams, each with its own speed and travel path. It therefore has two refractive indices, not one. Birefringence is the difference between the two refractive indicies. This is measured with a polariscope.

Nephrite’s birefringence is about 0.027; serpentine’s is about 0.014.

Few stones are singly refractive, so if you find one that’s singly refractive, you narrow down the list quite a bit.

Pleochroism

When a doubly-refractive stone splits light into two beams, each beam is absorbed differently by the stone. In many cases this difference in absorption is too minute to be observed, but when the difference in absorption is significant enough, the stone will show different (two to three) colours when viewed from different angles. This is termed pleochroism, and depending on if two or three colours are observed with a dichroscope, this tells a lot about the stone’s crystal structure.

Neither nephrite nor serpentine exhibits pleochroism, but other green-coloured stones (e.g. emerald) do–hence, process of elimination.

Absorption spectra

Spectroscopy is the study of matter interacting with electromagnetic radiation. The types and breadth of spectroscopic techniques available are enough to fill several textbooks, but for the purposes of gemology the most common type used is the absorption of visible light. The colour of a stone is due to what wavelengths of light it absorbs. If a stone absorbs all the wavelengths of visible light except for green, the stone will look green to the observer. The absorption of visible light can be seen on a device called a spectroscope.

I wasn’t able to find much information on diagnostic bands for nephrite and serpentine. Apparently bands can sometimes be seen at 510 nm and 495 nm for nephrite. Unfortunately, various types of serpentine seem to have bands at 540, 492, 495, 497-498, 464-465, 460 nm, so it would likely be hard to differentiate between nephrite and serpentine using a spectroscope. Other techniques (specific gravity, refractive index, etc.) are probably more useful.

Luminescence

When irradiated with electromagnetic radiation, some stones absorb that energy and release it so that the emitted energy is visible to the human eyes. UV is the most common energy source, though X-ray, visible light, and other sources are possible.

As far as I can find, nephrite and serpentine are not luminescent (but some other green-coloured stones may. Process of elimination again).

Note that UV-Vis spectra can also be observed using a more precise instrument called the spectrophotometer, which is capable of far higher resolutions than the human eye across a far greater range of wavelengths. However, to load a sample onto such an instrument does usually require some destruction of the sample. Also, spectrophotometers are quite expensive and usually out of reach of most gemologists.

Hardness

The Moh’s hardness scale ranks how hard precious stones are from 1 (talc, soft) to 10 (diamond, hard) and many precious stones have been assigned a hardness on the scale. However, since scratching the sample with reference materials will incur some damage, this is usually only performed on rough pieces or on inconspicuous areas, not on an obvious surface of a finished piece.

On Moh’s hardness scale, nephrite ranges from 6 to 6.5, while serpentine ranges from 2 to 5.5.

If your character is a gemologist, their options probably stop here (or pretty close to here). Again, a gemologist is a person trained to identify and appraise gems and precious stones, and tries to keep their sample undamaged. However, this blog is scriptchemist, not scriptgemologist, and chemists do have a few additional options. If your character is a chemist (preferably a materials chemist), with the infinite budget (hahaha…*sarcastic laughter*) and expensive instruments available to chemists, who are not limited to non-destructive options…well, the identification of this sample will probably go a lot quicker.

I will let J go into the details of the marvels of diffraction, as he is way more knowledgeable than I. :)

Numbers for the properties above were obtained from The Gemology Project or Wikipedia, and the absorption spectra were from The Spectroscope and Gemmology (not affiliated).

~Z

J here – As a materials chemist I would approach this problem a little bit differently, but that is mostly because (1) I am a crystallographer and (2) I happen to have access to a very particular instrument and an associated database, but we’ll get to that in a moment.

Let’s pretend your character knows a graduate student who happens to be a materials chemist like me. They show up with two stones, which might be samples from larger batches that have been sorted based on the criteria Z laid out above, or individual stones from some archeological dig, but your character needs positive identification. The first thing I would ask them is whether or not I can grind the stones up into a very fine powder. This may seem like an odd thing to do, but if the answer is yes I can probably tell them exactly what each stone is in about an hour, and they can have the powder back undamaged.

In materials chemistry a huge amount of the day-to-day work is something we call phase identification. While the word ‘phase’ is often used to refer to phases of matter (solid, liquid, gas, or plasma), materials chemists use the term to refer to a substance with a set elemental composition in a specific crystal structure. For example, titanium dioxide (TiO2) has eight different modifications with the same composition but different crystal structures; each one of these would constitute a different phase in a given sample. A researcher will perform a synthesis to get a material, but before they can do anything with it they need to confirm that it’s the exact material they wanted. In the case of TiO2 the different modifications have different catalytic activities, and having the wrong form could ruin your experiment. Elemental analysis wouldn’t be useful as the composition is the same for all the structures, and it may be very difficult or impossible to tell them apart spectroscopically as well. Because of this, the go-to technique for phase identification is powder X-ray diffraction (PXRD for short), because it is sensitive to both the composition and the crystal structure.

In a typical experiment a material is ground into a powder and loaded onto a sample holder – the more the better, but I’ve run samples with as little as 5-10 mg, which in this case would be roughly a 1.5 mm cube of either stone. A full description of how X-ray diffraction works is somewhat beyond the scope of this post (if anyone is wondering, start here or send an ask and I’ll tell you everything, it is literally my specialty), but suffice it to say that you shine X-rays on the sample and sweep a detector through an arc, measuring the diffracted intensity with respect to the diffraction angle. You end up with a discrete set of peaks, the locations of which tell you about the general symmetry of the structure and the intensities give information about the types of atoms and their locations within the structure. You get a different set of peaks for each phase present in the sample, but you can’t just look at a diffraction pattern and say, “yep, that’s a titanium right there, and a couple of oxygens over here” – that’s where the database comes in. The International Centre for Diffraction Data maintains a database with nearly one million distinct patterns of known materials, and the experimental pattern collected for each of the stones can be matched against the entries in the database.  As both nephrite and serpentine have been studied before, they’re in the database and their exact patterns could be matched fairly quickly for a positive identification. It might even be possible to narrow down a specific geographic location where the mineral was formed, due to natural variation in composition. This also works for identifying almost any mineral or crystalline substance – in my research, the only times I haven’t been able to match a sample to one or more entries in the database have been when I made something completely new.

Your character also might not need to befriend a materials chemist (but hey – we’re nice people! We like friends!), as there are private companies that specialize in phase identification and will analyze samples sent to them for a fee. It would be somewhat less plausible for your character to acquire the necessary equipment, as an entry-level benchtop powder diffractometer will set you back $80k, a five-year license to access the database is another $10k, and at least in the USA you’d have to deal with the state regulations and yearly inspections of equipment that produces X-rays – and the inspectors generally frown on having a diffractometer in a residential garage.

~J

Disclaimer

[acme2016]

The Story of Tanzanite part two

Tanzanite Jewelry and Gem Care

#Tanzanite is a gemstone that requires special care. With a hardness of 6.5, #Tanzanitegemstones can be scratched easily if brought into contact with other gemstones, like Diamonds for example. Make sure you store #Tanzanitejewelry in its own protective pouch to avoid scratching or chipping.

#CleaningTanzaniteJewelry is recommended every second or third wear. #AcmeGemJewelry recommends letting a piece soak for a few minutes in a gentle detergent solution and use a soft brush to clean around the setting. Pat dry with a soft, absorbent cloth, or allow to air-dry. If you take your Tanzanite in for professional cleaning, make sure they never, ever put Tanzanite into the ultrasonic machine and never let them steam clean it. You can check for more Tanzanites and Tanzanite Jewelry at https://acmegem.net.

 Crystal Meaning

The #CrystalMeaning of the exquisite #TanzaniteGemstone assists in the cleansing and opening of the #ThirdEyeChakra. This expands your awareness and creates clarity. The Tanzanite awakens your #intuition and spiritual sight.

·         Opens you up to #spiritualwisdom

·         Opens the third-eye chakra, #clarity, #vision, #imagination

·         Helps communication with the higher self and wisdom

#AcmeGemJewelry found that the violet light of #Tanzanite will illuminate your path so you can see clearly & get over your confusion & get rid of #mindclutter.

Tanzanite shows blue, purple and brown. It’s super cool to look at under a #dichroscope. If you have #acmegemjewelry do your #jewelryappraisers or your #gemidentification get the #acmegemgemologist to show you this!!

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Chelsea Filter, Dichroscope, Jewelers Loupe, Gemological Refractometer – Gem Identification Tools, Bundle 6 Items

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[mysaidi79-blog]

Understanding inclusions: part 4 tanzanite

New Post has been published on http://www.wholesalejewelrycatalog.org/business/jewelry-business/understanding-inclusions-part-4-tanzanite/

Understanding inclusions: part 4 tanzanite

Compared to other gemstones, tanzanite is young. JUNE MACKENZIE discusses this popular gemstone that Tiffany & Co brought to market in the 1960s.

What is tanzanite, the beautiful blue to violet- blue gemstone? Tanzanite ranges in colour from pale blue to pale violet-blue for smaller gemstones and a more saturated colour of blue or violet-blue for those larger in size. It belongs to the epidote group and sub-group called zoisite.

Although opinions may vary, generally pure saturated-blue tanzanite is the most highly prized in Europe. This colour is usually found in larger gemstones exceeding 4.00 carats; however, it may be also found in small gemstones from 1.50 to 2.00 carats. The US market tends to prefer the pure, saturated blue to have a touch of violet in it.

Tanzanite was originally discovered by gypsum miner Ndugu Jumanne Ngoma in Tanzania – Merelani, Arusha – in an area called Kiteto in January 1967. Ngoma wasn’t credited with this find until 17 years later when the Government of Tanzania finally acknowledged his discovery of tanzanite and presented him with a Certificate of Proof. During that long wait, Ngoma experienced many disappointments as other people were incorrectly credited with his discovery and even once this was corrected, he did not become wealthy as a result of his discovery.

Tiffany & Co named this exquisitely-coloured gemstone after the country in which it was found and began marketing it in 1968.

Zoisite comes in various colours including pink, green and brown. Brown is the most dominant colour and transforms into tanzanite when heat treatment brings out various shades of blue and blue/violet. Although zoisite/tanzanite can be found naturally in these desirable colours, the majority of tanzanite has its colour produced this way. The change in colour from brown to blue commences at approximately 350 degrees Celsius and so gem-quality gemstones are exposed to temperatures varying from 350 degrees Celsius to approximately 500 degrees Celsius for time varying from seconds to minutes. It should be noted that raising the temperature beyond 600 degrees Celsius can have a disastrous effect, turning the gemstone white and causing fracturing.

Tanzanite’s refractive index is generally 1.691 to 1.700 and the specific gravity averages 3.33. It is optically biaxial and trichroic, showing three distinct colours in a dichroscope – these trichroic colours change to different colours after heat-treatment. Tanzanite is relatively soft with a hardness of 6 to 7 on Mohs Scale.

Not much study has been conducted into inclusions in tanzanite; however, as no high- temperature treatment is carried out on the gemstones, there are rarely any changes to existing inclusions. Graphite and corrosion tubes that may intersect might be present (Figure 1), as may growth tubes, which never intersect, lamellar twinning (Figure 2) and some liquid inclusions.

The most highly-prized colour of tanzanite might theoretically be a pure, saturated blue but demand really is in the eye of the beholder. Colour is always a personal choice so sales dialogue should target whichever colour customers prefer.

When discussing after-sales care, remind customers that tanzanite is relatively soft so care must be taken when wearing it. It is not advisable for the owner or jeweller to clean it in an ultrasonic cleaner.

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[gemic-laboratory]

Ametrine pair 11.43 ct. Click The link buy now: https://bit.ly/3zbxfVG . . . . #ametrine #ametrinecrystal #ametrinestone #ametrinejewelry #bolivianite #ametrinegemstone #gemology #gemmology #fieldgemology #fieldgemmology #gemmologist #gemologist #gemologybook #book #gemstonebook #gembook #gemmologybook #gemeducation #gemologytools #darkfieldloupe #dichroscope https://www.instagram.com/p/CT5LIZMBV2C/?utm_medium=tumblr

[acme2016]

#TheStoryofTanzanite

“Buy the Ticket Take the Ride”

Hunter S. Thompson

How about if you hop on the bus with #AcmeGemJewelry and take a little ride back to the past and discover with #AcmeGem the journey of #Tanzanite.

About a decade ago, in 1957 a bolt of lightning started a wildfire which engulfed the #MerelaniHills in #Tanzania. The fire ripped through the land, leaving it barren. However, when the fire had subsided the local #Masai herders noticed brilliant blue #Tanzanite crystals laying around everywhere.

The #TanzaniteGems that they had discovered held a secret. #TheSecret was that the #TanzaniteGemstones they found only became blue under scorching heat. #RoughTanzanite is brown and drab until it is heated and then it becomes an amazing blue/purple/brown gemstone.

Ok…hold onto your seats and let’s continue your trip with #AcmeGemJewelry.

Pretty soon a fellow by the name of #ManuelDSouza came along and thought the #BlueGemstones were #Sapphires – which of course, they were not…but it took a lot of jewelers and #Gemologists quite awhile of researching until they realized this was a completely #unknowngemstone and the #BritishMuseum and #HarvardUniversity decided to call it #BlueZoisite. Tanzanite got current name from #TiffanyandCompany.

 About the #GemstoneTanzanite

#BirthstoneforDecember, Tanzanite is a gem that suits any personality. Pretty well all Tanzanites are #heattreatedtanzanites otherwise they would be brown. The gemstone is #trichroic meaning that it shows three colors in different directions. Tanzanite shows blue, purple and brown. It’s super cool to look at under a #dichroscope. If you have #acmegemjewelry do your #jewelryappraisers or your #gemidentification get the #acmegemgemologist to show you this!! Don’t worry the #gemstoneheattreatment in Tanzanite is permanent so it doesn’t disappear with polishing or cutting and does not affect the #priceofTanzanites.

#Acmegemjewelry, #MerelaniHills, #Tanzania, #TanzaniteGems, #TanzaniteGemstones, #RoughTanzanite, #AcmeGem, #BlueGemstones, #sapphires