Crystals are repeated patterns of minerals with sharp corners and flat sides. Different types of minerals have different crystal structures based on the types of elements they are made of and the density of those particles.
The repeated pattern of particles within a crystal give them specific properties that can be used to identify them.
Crystals can be identified by, color, hardness, density, and luster - as well as other factors.
A quartz crystal can be identified by a light color, hard surface, low density, and glassy luster.