I came up with a kit of two textures which can be used to represent any single color gem. All you do then is apply a self illumination color at about 128 luminance. If you don't want it to glow, add the same color to diffuse/ambient instead. You have a second color choice which affects only the crystal outline, which nothing but a 1cm sheathe around the entire crystal. I find that a combination of similar colors looks good, like red/magenta, but other color combos such as red/blue look good. Basically pick a partner that is only one or two major hues away on the color wheel. The outer sheath self illumination barely affects the color of the underlying crystal, but does add a tiny amount of hue change when the sheath picks up the brightest envmap regions. It isn't powerful enough to give a major two-hue crystal though, like you might see with real ruby.
You can combine three colors by using both self illumination color and diffuse/ambient on the same crystal, however, like with the second self-illumination, the color is barely noticeable. Using a base hue of yellow with a self-illumination of wine red (128 red) still gave a very dark red-orange hue. Again the second-self illumination, even when set to super bright, does not affect the overall crystal color in any really noticeable way, but the brighter the color, the brighter the outline.
If you give all three the same color, or four if you count the amb/diff on the upper layer, the crystal can be made really vibrant. For a test, I used bright cyan as the self illumination color, 256 blue as the amb/diff, and bright cyan again as the upper self illumination. It was both very vibrant (but not white) and sparkly (but not metallic)
If the opacity on both the inner and outer crystals is set to 50%, and the self-illumination on both is no higher than 128 luminance, you can easily achieve a crystal of one color which flickers in another. In that example, I find that colors even three major hues away make great combinations, such as yellow/magenta.
The above picture is a dark yellow under-crystal with a magenta over-crystal. It shines yellow and flickers in wine colors.