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 Post subject: Identifying rough gems
PostPosted: Sat Feb 07, 2009 8:23 am 
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Hi there everyone

I really battle with identifying rough gems as I can't seem to grasp the idea of crystal systems and habit, no matter how many times I read my FGA notes, various gem books and online articles on the subject. I understand the laws of the crystal systems but I can't seem to "see" the crystal system in a gem. Are there any guidelines to working it out or is it just something that needs lots of practice?

I'd really appreciate any help as this is a pretty big problem for someone studying gem identification!!

Many thanks
Gem (- seriously!)


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 Post subject:
PostPosted: Sat Feb 07, 2009 10:11 am 
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Well, sometimes you do have to look very carefully at a rough gem to find some of it's original crystal surfaces to identify. And sometimes, there just isn't any of the original crystal surface left to see.

For example, you would not be able to identify this piece of garnet rough based on crystal shape, because none of the original crystal shape remains. (However, the conchoidal fracture is one clue that this is in fact garnet... but glass also has conchoidal fracture, so you have to be careful!)
Image

These garnets are easy to identify based on their characteristic crystal habit:
Image

One caveat though - there are people who will carefully machine gems to look like other types of mineral crystals... a classic recent example is phenakite that was machined to look like diamond crystals! (Click on the image to go to Thomas Hainschwang's full report.)
Image

The lesson - use a preponderance of evidence to help you figure out what minerals you have! :)

You might try looking at some mineralogy books for more help with learning how to ID gems using crystal systems, fracture, hardness, etc. Mineralogy relies heavily on these characteristics for identifying minerals in rocks.


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 Post subject:
PostPosted: Sat Feb 07, 2009 1:58 pm 
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I find it rather confusing sometimes too. Let's not forget mineral epimorphs, pseudomorphs, multiple crystal systems and habits for the same mineral.....

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These gems have life in them: their colors speak, say what words fail of.


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 Post subject:
PostPosted: Sat Feb 07, 2009 2:38 pm 
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Hi,

Think of the different forms that make a "habit" as lego tiles (the "forms") with which you can make different shapes (the habit).

Image

With a lot of cubes you can build 2 pyramids (as they did in Egypt), glue them together at the base and you have an octahedron .. one of the shapes (or better "habits") of the cubic system.

Hope that helped a bit.

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 Post subject:
PostPosted: Tue Feb 10, 2009 12:21 pm 
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Hi there

Thank you all so very much for your help. I guess not putting so much emphasis on the crystal system is what I can take away from this.

Thanks especially to Doos, I didn't think I'd ever understand habit and form so clearly!! They make so much sense now.


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