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Crystal Chemistry
Crystal groups are classified by the chemistry composition of the mineral. There are 15 classes: native elements, sulfides (includes arsenides, tellurides), sulfosalts, simple oxides, hydroxides, multiple oxides, halides, carbonates, borates, sulfates, chromates, phosphates (includes arsenates, vanadates), vanadium oxysalts, molybdates (includes tungstates), and silicates. Fortunately, for those of us who are not chemists, many crystals can be identified from observable features without the necessity of knowing the details of the chemical composition.
Crystal Form
Crystal "form" refers to the shape of a crystal. Crystals always form according to geometric mathematics. There are six basic shape classifications that all crystals fall into: isometric, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. Because there are thousands of crystal types, shape alone cannot be used to determine a mineral, but shape can be used to narrow down the possibilities. For example, a yellow fluorite crystal and a yellow halite crystal are both isometric, often cubic, and more features must be determined to tell the difference between the two. Crystal form can also vary within the six classifications. For instance, garnet is isometric, but crystals usually are dodecahedrons or trapezohedrons.
Hardness
Hardness is a feature that determines the crystal's use as gemstones. Some crystals are extremely beautiful, but are not hard enough to be sufficiently durable for faceting into gemstones. Soft crystals are often used as cabinet specimens instead. A crystal's hardness is graded on a system called Mohs scale of hardness. This scale runs from one to 10. Diamonds are the hardest mineral, ranking 10 on the Mohs scale. Talc is the softest mineral, registering only 1 on the Mohs scale. A crystal can only be scratched by a harder crystal, so hardness is often used as an indicator of crystal type.
Other Identifying Features
Other features of crystals are used in combination to identify minerals. While many minerals have similar colors and other features, the combination of features will be unique in a mineral. Luster refers to how a mineral reflects light, that is, whether it is transparent, metallic or silky looking, or even opaque. Fracture and cleavage show how specific minerals break along flat faces or chip when struck. Gravity refers to weight of the mineral. Streak is the color of the residue left when the mineral is rubbed on a streak plate. Once all features are considered together, the type of mineral the crystal is composed of can be ascertained, most of the time without need of laboratory identification.