Identifying gems by color

[zeolite]

Certain colors seem only to belong to one gem species, and it can be a mistake to try identify unknown gems by color alone.

Please forgive the picture below. The camera recorded the center stone as more pinkish than the other two. To my eye, the center stone is much closer in color to the other two than shown here.

The left stone is an extremely intense citrine from Brazil. The right stone is a spessartite garnet from Nigeria. The center stone was supposed to be a tourmaline from Mozambique. I had considerable doubts about it being a tourmaline, because after 25 years of gemming, I''ve never seen a tourmaline in a reddish-orange color, without huge amounts of brown. I''ve seen golden, golden-orange, red, pink, and copper, but never a fairly saturated reddish-orange.

I inspected the gem in a microscope and saw facet doubling, so that eliminated garnet. I measured the refractive index at 1.620, 1.640, so it is tourmaline! Suppliers in the U.S. are quite reliable, but some overseas sources could be less reliable, so I thought it was wise to check.

I now have a totally new color in my tourmaline collection!

 

[Pandora II]

Very pretty stones.

You have a lovely collection. Can you tell us more about your background?

 

[zeolite]

BS, physics & math
Gemologist (GIA) 1983
retired, 4 patents in electro-optics at DuPont

https://www.pricescope.com/community/threads/test.92731/
An article on these two color change tourmalines, pictured in this thread, is published in Gems & Gemology, Fall 1991, page 184

https://www.pricescope.com/community/threads/thoughts-on-this-garnet.92407/
Click on yellow grossular suite link, see square Barion, upper left. This gem was described in Gems & Gemology, Winter 1991, page 258

Dr Jim Shigley, who was head of GIA research at the time, was kind enough to chemically and optically analyze and publish articles on these unusual gems.

 

[Pandora II]

Thanks and welcome to PS - I hope you stick around.

Are tourmalines and garnets your main area of interest? Where do you find your stones?

Sorry, lots of questions!


I''m merely a consumer, but in the middle of studying for my FGA and I have a terrible garnet addiction. I don''t have very many as yet as I prefer to save up and buy nice stones.

 

[zeolite]

I could answer in considerably more detail, but not in a public forum. Is there some system here for private messages?

 

[Pandora II]

There''s no such system here I''m afraid.

 

[zeolite]

O.K, here''s the condensed version:

I collect everything, including rare collector stones that are too soft to wear. My favorites are garnet and tourmaline, because there is huge color variety in these gems, and they are affordable in larger size and finest quality.

I buy some stones in Tanzania and Burma, but mostly in U.S.

So you live in Great Britain, and are studying for your gemology degree?

 

[Pandora II]

Date: 8/17/2008 6:02:02 PM
Author: zeolite

O.K, here''s the condensed version:

I collect everything, including rare collector stones that are too soft to wear. My favorites are garnet and tourmaline, because there is huge color variety in these gems, and they are affordable in larger size and finest quality

I buy some stones in Tanzania and Burma, but mostly in U.S.

So you live in Great Britain, and are studying for your gemology degree?

I''m based in London, I''m doing the FGA via correspondence though as I work full-time. I have a great tutor in Birmingham, which is where my husband is from, so we try to meet when I''m up there visiting family to play ''Name that Stone'' using loupe only. It''s a lot of fun, but means that I do try to add the odd rareity to my collection in the hopes of stumping my tutor.

We do a lot of work on synthetics as well - it''s a big part of the course in the stone ID exam.


Other than my engagement ring and pearls I don''t really wear jewellery, so almost all my stones are kept in boxes - which means I''m happy to buy the softer ones (I do confess to setting some sphene, but I don''t intend to wear them very often).

Garnets are by far my favourites - as you say, you can buy fine quality in good sizes and not break the bank! I only wish someone would bring out a new definitive garnet book. I have ordered all the GIA articles on them, and I''m still trying to get my paws on Bill Hanneman''s book - Rouse is still too pricey, but I have a call-out on it with a lot of UK book-dealers so I may strike lucky.

I''m buying nearly everything in the USA - I''m not finding a lot that interests me in the UK and the prices are ridiculous. I am going out to Sri Lanka in October and I''m hoping to pick up some nice pieces there if I can - hence all the stone ID practice!

 

[innerkitten]

Hi, In another post you had a group of pink garnets that were very pretty. What were they?
TIA

 

[zeolite]

I’ll give you a short answer and a long answer.

Four are pyrope garnets and one is a rhodolite.

As Richard M. kindly pointed out, while there many garnet types known to geology, only about five are seen in jewelry: pyrope, almandite, spessartite, grossular and andradite.

Pyrope is a magnesium aluminum garnet
Almandite is an iron aluminum garnet
Rhodolite is a name for a garnet, which is a mixture of pyrope and almandite

Pink garnets are quite rare. I would estimate that for every 1,000 red garnets, there is fewer than one pink garnet. Pink garnets are rare, because the principal coloring element in red garnets is iron. Iron is very abundant in rocks worldwide, and the normal garnet is a deep red.

There are very few places in the world that is deficient in iron. It is only in these places that a garnet can be low enough in iron to be pink. One such place is northeast Tanzania, because in Prehistoric times, it was an ancient seabed, and the ocean floor was littered with calcium from the shells of sea animals.

I may have the world’s foremost collection of pink garnets. I know I’ve bought about every attractive one that has been offered. A pink garnet has a lower tone, and lets more light reflect back compared to a red garnet. So a properly cut pink garnet, combined with the high refractive index of a garnet, can provide amazing brilliance. It is a poor person’s pink diamond, providing much larger size and much lower cost.

The garnet below, is the largest size and lightest tone, of the 5 pink garnets pictured in my previous post. It is a 15.92 ct pink pyrope, cushion cut, by the person I consider the finest cutter in the world. I bought it about 15 years ago from an estate. It was originally purchased about 30 years ago.

 

[Pandora II]

That is a gorgeous stone.

Do you have any finer details - ie RI, chemical comp. etc?

Since you are into the pink garnets, just out of interest what do you know about ''rosolite'' garnets - do you have any or any info on them?

 

[innerkitten]

It''s lovely! Thanks for the information. Feel free to post more of your collection if you ever have some spare time

 

[zeolite]

I seem to recall the R.I= 1.735 and very faint iron spectrum at 505nm. I follow the Stockton - Manson 1985 classification, and the low R.I. and pale tone indicates unusually high pyrope content. The one pink garnet of the 5 that I pictured that classfied as rhodolite, just barely squeaked over the R.I=1.742 dividing line to become rhodolite.

AS far as the Mexican Rosolite, I am not aware of any of the material being facet grade, i.e. perfectly transparent.

 

[movie zombie]

i''ve never been one for pink stones, but that pink garnet is one i''d want! nice cutting.

movie zombie

 

[zeolite]

I was interested in your comment, not caring for pink. I opened a new topic below, to keep this thread from straying too off course.

https://www.pricescope.com/community/threads/hot-pink-sapphire.93168/

 

[Pandora II]

Date: 8/20/2008 7:26:12 PM
Author: zeolite
I seem to recall the R.I= 1.735 and very faint iron spectrum at 505nm. I follow the Stockton - Manson 1985 classification, and the low R.I. and pale tone indicates unusually high pyrope content. The one pink garnet of the 5 that I pictured that classfied as rhodolite, just barely squeaked over the R.I=1.742 dividing line to become rhodolite.

AS far as the Mexican Rosolite, I am not aware of any of the material being facet grade, i.e. perfectly transparent.

Thank you, that is very useful.

I don''t supposed you''d be willing to post pics of all 5 and their RIs and spectrums? Do they follow a depth/tone of colour pattern as the RI increases?

 

[zeolite]

That will take a little while, to re-photograph them individually, which I need to do. R.I is no problem. But I don''t have the means or proper lens to photograph the inside of my spectroscope. I can view the spectrum, but not photograph it.

 

[Pandora II]

Date: 8/20/2008 9:18:34 PM
Author: zeolite
That will take a little while, to re-photograph them individually, which I need to do. R.I is no problem. But I don''t have the means or proper lens to photograph the inside of my spectroscope. I can view the spectrum, but not photograph it.

If you can approximate the numbers for the bands that would work for me as far as the spectrum goes.

Thank you very much - I look forward to seeing them!