Hardness
The hardness of a gem or crystal can help with identification, according to the College of Natural Resources at Berkeley University. The Mohs scale rates the hardness of a mineral, ranging from 1 (soft substances, like talc) to 10 (hard minerals including diamonds). By scratching unidentified gems with known minerals, identification becomes easier. For example, if a gem can be scratched by quartz, then it has a hardness level of less than 7 on the Mohs scale--because that's the hardness of level of quartz. That means every type of gem above that level can be ruled out as a possible ID.
Color Streaking
By itself, color is not a particularly useful way of identifying gems and crystals. Even gems of the same type can have different colors depending on any chemical impurities, according to Rocks Fork Kids. A more reliable method is using a color streak. A small amount of gem dust, or the gem itself, is smeared against a pale ceramic tile. This sometimes leaves a colored mark. The resulting color can be compared to a color streak chart to find out if it's a metallic or non-metallic mineral, as well as narrowing it down to likely gem types.
Luster
Luster refers to the way a gem or other mineral reflects light on its surface. It can be used to help determine crystal gem identification, according to the University of North Carolina at Charlotte. Luster appearance is divided into the categories resinous, metallic, silky, dull, glass-like, adamantine and pearly.
Shape
Minerals that grow into full crystals are usually easier to identify by shape. For example, some crystals develop into sharp needles, some grow into layered "leaves" of crystal, some make rough cubes and some grow into hollow geodes. A crystal identification chart can be used to check the shape and ID of a mineral.
Cleavage and Fracture
The way that a mineral breaks reveals a lot about its true identity. Cleavage can be a destructive test, but it's a very useful way to determine gem type. Identification is based on whether the gem splinters, or cleaves into two flat surfaces. If the mineral breaks in an uneven way, it's referred to as a fracture. You should take note of the fracture edges--including whether rough, smooth or jagged. This can help in finding a gem's identity.