|YAG boule top.
YAG stands for yttrium-aluminum-garnet: It's used to make lasers and can be doped with various elements to create different colors. This green one, for example, is doped with chromium.
The conical shape is similar to that of my silicon boule top, presumably because they are made by similar crystal-growing procedures. A crystal begins growing from a seed, expands out until it reaches the diameter of the melting pot is being pulled from, and then continues to grow as a straight-sided cylinder as it is pulled slowly out. Only the full-diameter portion of the rod can be used, so the cone at the start becomes waste and ends up with someone like me. (Actually, even this waste is quite valuable, probably because it can be melted down and reused. But Max found a nice company willing to sell us the cutoffs for a reasonable price.)
Source: Max Whitby of RGB
Contributor: Max Whitby of RGB
Acquired: 10 October, 2003