What type of gemstone is created by fusing two materials together?

The correct choice describes a "doublet," which is a type of gemstone created by fusing two different materials together. This technique usually involves layering materials such as a natural gemstone and a glass or synthetic material. The purpose of creating a doublet is to enhance the appearance, durability, or affordability of the gemstone. For instance, a doublet can provide the look of a more expensive stone while maintaining some essential characteristics through the backing material.

In contrast to the doublet, a composite stone typically involves the combining of two or more pieces of material that are not necessarily layered to enhance the overall look but rather to create a single piece from several parts that might be glued or mechanically bonded.

A brilliant cut refers to a specific cutting style that optimizes a gem's brilliance and sparkle but does not involve the fusion of different materials, while a faceted gem simply refers to a gemstone with flat surfaces (facets) that reflect light, regardless of how it was formed. Neither brilliant cuts nor faceted gems inherently involve the combination of materials in the same way that doublets do. Hence, the distinctive characteristic of a doublet is its fusion of two disparate materials to form a single piece, which categorizes it correctly in this question.