Doublet (lapidary)

A doublet is a type of assembled gem composed in two sections.

A garnet and glass doublet uses a top portion of natural garnet fused to any color of glass to imitate a gem.

A harder garnet makes the stones more durable.

In the case of opal doublets, a backing layer of onyx or matrix (ironstone) gives the more fragile opal layer support and can make the opal look darker and higher quality.

They were still being produced into the early 1900s until actual synthetic gems were introduced.