Post subject: Re: Why do hydrothermal synthetics look so weird?
Posted: Sat Jun 27, 2020 11:19 pm
Gold Member
Joined: Thu May 12, 2016 2:18 am Posts: 1542
They have crystal structure just like the natural stones. They are not microcrystalline.
The Russian Hydrothermal rough looks like slabs because of the seed crystal growth method. This method leaves a colorless, often near opaque core in the rough. The rough pieces are trimmed usually by cutting them down the middle to remove this unusable core. This is why they look like slabs. If you saw them before being sawn you would see the crystal shape.
Post subject: Re: Why do hydrothermal synthetics look so weird?
Posted: Wed Jul 08, 2020 5:52 pm
Gold Member
Joined: Mon Apr 16, 2007 12:05 am Posts: 1499
Mandira wrote:
Why do they all have that weird slab shape?
Mandira is asking why raw synthetic emeralds have the "weird" slab shape.
The short answer... When you arrange crystallization correctly, a slab is the best shape for arranging the best color to align with the surface that provides maximum yield when faceting.
The long answer... This requires a little background on emerald color. To summarize Basil Anderson from his "Gemstones for Every Man" 8th edition, p 174: The dichroic nature of emerald creates a bluish-green color along the extraordinary ray direction and a jade-green color along the ordinary ray direction. Many people (including both Basil and I) prefer the jade-green color, so we'll describe this as best color.
But you can decide for yourself... the photo below shows (clockwise from the left) natural crystals from Colombia, Zambia, and Afghanistan. Notice the hexagonal prismatic crystal habit.
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natural emeralds.jpg [ 65.05 KiB | Viewed 2426 times ]
The crystal habit is important. The color seen through the hexagonal face of each crystal is the isolated jade-green color, whereas the color seen through the prism sides is a mix of bluish-green and jade-green colors.
Now if you wanted to facet one of these crystals for best yield, you'd choose the gem's table to lie along a prismatic side. So after faceting to produce some kind of emerald cut, looking through the emerald's table, you'd see the less desirable mix of green colors.
Faceting to produce best color, you'd have to orient the gem's table along the hexagonal face. And then you'd end up with a single round brilliant with great color, but that doesn't use much at all of the stone's depth.
So you could have either a single 1 carat emerald with poorer color, or two 0.3 carat emeralds with great color.
Synthetic emeralds can be grown so they reproduce this natural crystal habit. Below is a picture of a Chatham-created cluster of hexagonal prisms. (Note that this is a quick photo taken by phone under less--than-perfect lighting, so the different greens aren't distinct)
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But mostly synthetics are grown in slabs as Mandira linked to, with crystal growth oriented so that the best color is seen through the wide surface of the slab. Looking at the slab in the linked image, it is clear that the jade-green color is seen along the wide surface, whereas the narrow ends have a bluish-green color.
I have a piece of such a slab (again, not a good phone photo)...
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flux.jpg [ 48.55 KiB | Viewed 2426 times ]
Let's look at a couple pictures of the slab seen through a dichroscope:
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dichroscope view.jpg [ 49.38 KiB | Viewed 2426 times ]
Clearly, the wide surface produces a single color, whereas a narrow side produces two colors. Arranging a gemstone's table along the wide surface will produce the best color and the best weight retention.
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