A composite gemstone consisting of two pieces of the same or different gem materials, or one gem material and a second of glass.
An example of a doublet. This could be a natural, undesirable color sapphire crown on a deep blue synthetic sapphire pavilion. Due to 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 which are diagnostic for natural material thus convincing an unweary gemologist that he/she is looking at a natural stone.