Split Boules of Flame Fusion Corundum. Photo by Morion Company.

Boule refers to the rough form of synthetic corundum or spinel, produced through the flame fusion/Verneuil method, which is either pear or carrot-shaped. The name comes from French (“ball” or “globe”) and was applied to the globular shape of the first successful laboratory-created crystals. Newly created boules are usually split in half to relieve the crystal of some of its strain.

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