looks like it could be murkiness but the pic is so small I can't say

int he transparency tab, Murkiness sets the effect of oh, cloudy, dense materials, from dirty water, to gems or glass that's semi-opaque. Higher the value, more opaque it gets.

Also note, for many gemstones you MUST have that turned on, by the way as they aren't perfectly transparent. Many gems are indeed murky, especially when large or poor quality
:)

here I turn emerald, which isn't normally anything so storngly murky, way up, making a nice interesting material for crystal balls etc.

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