Which characteristic is specific to jet as a gemstone?

Jet, a type of lignite coal, is known for its distinct properties that make it recognizable and unique among gemstones. One of its key characteristics is its glossy finish, which can result from the smooth carving and polishing of the stone. This glossy appearance enhances its aesthetic appeal, especially when crafted into jewelry pieces like beads or cameos.

While jet can exhibit some brittle qualities due to its composition, it is not primarily defined by its brittleness. Its weight can vary, but it is not particularly heavy compared to other gemstones; its lightness is actually a noted aspect. Additionally, jet is typically opaque rather than translucent, which differentiates it from various other gemstones that allow light to pass through.

Thus, the glossy finish stands out as a defining feature of jet, setting it apart in the realm of gemstones.