A composite gemstone consists of two pieces of the same or different gem materials, or one gem material, and a second glass.

An example of a doublet would be a natural, undesirable color sapphire crown on a deep blue synthetic sapphire pavilion. Due to the refraction of light, this stone would appear blue all over when viewed in person. Immersion in a liquid with a high refractive index will reveal the different colors of the top and bottom of the stone. The natural crown may contain inclusions that are diagnostic for natural material thus convincing an unwary gemologist that he/she is looking at a natural stone.