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 it's strain.