A doublet is a type of gem composed in two sections. It is sometimes used to imitate other, more expensive gems. A garnet and glass doublet uses a top portion of natural garnet fused to any color of glass to imitate a gem. The color of glass used in the doublet is all that is seen, the garnet provides no color. If seen in reflected light, a separation line may be seen. The 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
