Bah, I had a really good tutorial in my favorites folder...now it's gone. =( Anywho, Here's a crash course in Tau optics:
Make sure the gem is painted black before hand. In the lower-left hand quadrant of the circle, throw down your darkest color you wish to be using. For example, if you're making a green gem, probably use Dark Angel's Green or a mix or Chaos Black + your green. After the darkest coat is down, you keep adding up layers on top of the dark color, getting lighter and lighter with each layer. You'll want to leave a little bit of the previous layer showing to get that "blending" effect. Essentially you're just working your way back to the edge of the optic, getting lighter and lighter each time, until you reach something that's close to white along the very edge. If you want a duller optic, you can leave it be, or if you want a sharper optic, for the final highlight add in some Skull White along the very edge. Finally, put a tiny little dot of Skull White on the opposite end of the circle where you've blended. That'll give the illusion that it's a convex surface with a shine, even though it's a concave surface! Ideally, you'll want your optic to have:
-A white dot at the opposite side of the circle where you've blended
-A little black "buffer" zone in between your white dot and your blending
-An even distribution of color in the blend
-Another shine on the edge of the circle with the blending to counter the dot.
For less than 6 month ago there was an article on this same subject in White Dwarf, if you have access to that I recomend you have a look there too.
The article listed here is good too, but I recomend using more than one source to get the best ideas.
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