Jewels:
Normally light falls on a round ball like this

But on a jewel the light goes through it. The same basic idea
applies to the iris of an eye or anything else that light can go
through.

The first thing you do is draw or color your jewel with a base
color

Then you add some shading on the side of your light source (Or
you can start with your dark color as the base and add a lighter
color to the side opposite the light source. Either way works)

Now add a highlight that makes sense with where your light
source is. Make sure it's lighter than the base color you used.

Then a little white highlight and it's done
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