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Here's where I'm at with the material so far:

http://i.imgur.com/vaw865e.png

I've been trying for the past couple weeks to model some Amethyst and have been banging my head against the desk so to speak. The problem I'm having is in modeling the volumetrics accurately. In most pictures I can find of amethyst it starts at the bottom of the crystal growth as a dense white material moving into a light purple, becoming more densely purple near it's tip. See below:

http://www.gemselect.com/other-info/graphics/brazilian-amethyst-rough.jpg

Here's my problem. I've found that no amount of volume absorption, or volume scatter set to white at ANY density looks right. I've tried them separately and I've tried combining them with an add shader. The volume scatter always looks too much like smoke even at very high densities and I can't really seem to get the effect of very dense white crystal that is apparent at the bottom of most amethyst.

Right now, the way I'm achieving the white bottom on the crystal is to blend between a diffuse shader and a glass shader using a gradient texture that I control by a vector mapping node to get it lined up just right. The problem with this is that a real amethyst crystal would still be a crystalline material at the bottom, albeit far too dense and white to see through. Volume scatter and absorption techniques didn't seem to allow me to achieve this so I'm sort of faking the white bottom, but I would definitely like the white bottom of the amethyst to look as realistic as possible in my amethyst material.

Some help would be greatly appreciated!

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  • $\begingroup$ Related - blender.stackexchange.com/questions/30609/… $\endgroup$ – Mr Zak Nov 9 '15 at 18:35
  • $\begingroup$ There is a very good chance that the lighting rig is a VERY important element of your source picture. Where was the lamp providing the white light? Maybe the dark purple section had to rely on light from a less direct source. $\endgroup$ – Mutant Bob Nov 9 '15 at 20:00
  • $\begingroup$ also related: blender.stackexchange.com/questions/40226/… $\endgroup$ – cegaton Nov 9 '15 at 20:04
  • $\begingroup$ I actually already am using the trick with adding red green and blue glass nodes :-P I still don't see anything in either of those links that would help me with the white crystalline bottom you see in any amethyst growth EDIT: Btw, I'm using an HDRi for lighting. I think for it to look the most convincing I would somehow need to make the white part using volumetrics, which I can do, but I've never been able to achieve a pure white mineral using volume scatter. High densities always come out looking a bit grey, rather than brilliant white. $\endgroup$ – PrismaPhonic Nov 9 '15 at 20:36
  • $\begingroup$ ok, making SOME progress. I found that with the glass node once it's applied to the surface any white absorption or scatter nodes attached to the volume of the material become black! I found out this is from SHADOWS. A work around I discovered was to use a mix shader to mix between glass and transparent shader, with the shadow ray light path controlling the mix factor. This allowed me to finally get WHITE volumetrics with a glass shader (although, not quite as brilliant white as I would like, but still, progress) $\endgroup$ – PrismaPhonic Nov 10 '15 at 0:07
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After a lot of tweaking, I came up with a result that looks decent: Amethyst gem render

Or how it is more likely to be: Amethyst rock render

I used a different reference so it's more purple than yours, but that's easily fixed since you seem to have no problem doing the actual crystal material.

Here is the node setup: Nodes

I'm sure you know what the top box does since you have already done that. It simply transitions from the transparent crystal material to the opaque white material.

The white material's surface is just a simple glossy add to a transparent shader. I have added it so as to not block any of the light going into the crystal.

I have separated out the purple crystal material into Refraction and glossy so as to have more control. I needed to blur the reflections slightly without adding roughness to the refraction.

The volume scatter is set to white with a really high density (around 50).

The real trick though, was to go to the Light paths and increase the volume bounces from 0 to 5. This causes the light to bounce around and some of it to come back out, causing the volumes to be a lot brighter.

Volume bounces

This however increases the render times exponentially. But if you're going for realism, then it's a trade you have to make.

Note: I haven't added a scratch texture to this so the reflections are perfect at the moment. But I have included a procedural scratch texture in the attached blend file, but it isn't connected to the material. It isn't very good but works if it isn't going to be viewed close up.

Hope this helps you!

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