What term is used for a cylindrical synthetic crystal produced by the flame-fusion process?

The term "boule" refers to a large, cylindrical synthetic crystal created through the flame-fusion process, which is a method commonly used in the production of synthetic gems, particularly for corundum (sapphires and rubies). During this process, aluminum oxide is melted down and slowly cooled to form a large, single crystal. The boule is then cut into smaller pieces or gemstones for use in jewelry.

This process allows for the creation of high-quality synthetic stones that can closely mimic the appearance of natural gemstones. The term "boule" is specifically used in the context of synthetic gem production and denotes the form that the crystal takes before it undergoes cutting and polishing.

Other terms in the choices refer to different concepts in gemology. Brilliance denotes the amount of light reflected from a gem's surface, the brilliant cut describes a specific style of cutting that enhances a gem's sparkle and light performance, and cabochon refers to a method of cutting gemstones into a smooth, rounded shape without facets. Each of these terms is relevant in the context of gemstones but does not describe the cylindrical crystal produced by the flame-fusion method like "boule" does.