1. A crystal is a solid material with a regular internal arrangement of atoms. Because of this orderly composition, it may form the smooth external surfaces called faces that allow us to see into the crystal when it is clear. Most all stones are made in part of silica. The presence of this silica is what gives crystals their luminosity and crystal clearness. Crystal is brittle—as we are—and as such is a reflection of ourselves. As we shatter our being, it is seen to be rigid and crystalline in structure. Crystals are described as being clear, milky, having rainbow prisms within, or having fractures visible along their length.
SOURCE: Truth about Crystal Healing, Phyllis Galde
2. Types of stones given value by their beauty of color, clarity, and shape. The term is usually attributed to those stones of color that are not clear quartz by those who use such stones for mental, physical, or spiritual healing and attunement. Each gem is associated with various qualities or powers determined by historical attributions (amethyst, for example, is said to prevent drunkenness), color magic, or modern research and experimentation.