Chemical formula: The chemical recipe for a specific substance. For example, the chemical formula of water is H2O, telling us that each molecule of water has two hydrogen atoms (H) and one oxygen atom (O). The mineral quartz has the chemical formula SiO2, so each molecule is made up of one atom of silicon (Si) and two atoms of oxygen (O).

Cleavage: The tendency of a mineral to break along planes of weakness in its crystalline structure.

Crystalline structure: The atoms or molecules in the substance are arranged in a repeating, three dimensional pattern.

Fracture: Any kind of break that is not cleavage.

Inorganic: Not part of a living thing or produced by a living thing.

Luster: Luster is the way that an object reflects light. Common lusters include metallic (looks like metal), vitreous (looks like glass), and earthy (looks like dirt).

Metallic luster: Things that have a metallic luster look like they are made of metal.

Mineral: A naturally occurring, inorganic solid, with a definite chemical formula and an internal, crystalline structure.

Solid: Not a liquid, gas, or plasma. A state of matter that keeps its size and shape, regardless of its container.

Vitreous: Glassy. Things that have a vitreous luster reflect light like a piece of glass.