Composite stones are composed of more than one part. Historically, composite, includes two basic types which are doublets and triplets. The doublets are composed of two parts, sometimes held together by a colored bonding agent. Triplets are composed of three parts, usually glued together to a colored middle part.
Doublets are widely used in making antique jewelry. In antique jewelry, the most common encountered doublet, is referred as “false doublet”. It consist of a garnet top fused to an appropriate colored glass bo0ttom. Garnets were used for the top portion of these false doublets.
True doublets are created by using two genuine stone parts and fusing them together with an appropriate colored glue to create a larger gems, better than the original component.
Opal doublets also occur, consisting of a thin top layer of genuine Opal cemented to a base that can create a poor grade of Opal. The most commonly encountered Opal doublets are precious black Opal.
Triplets are frequently encountered in the Opal market and, because of Opal increasing rarity, it has replaced the doublet. The triplet is like the Opal doublet except that it uses thinner Opal layer that is afterward covered by cabochon shaped colorless quartz.
There is nothing wrong in buying doublet, but you have to be very careful while buying Opal gemstones.