Crystal Habit.

Crystal habit is the word mineralogists use for the typical shape a mineral grows into when nothing crowds it. Each species has a preferred internal arrangement of atoms (its crystal system, of which there are seven: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal), and when a crystal forms with enough room, that arrangement shows itself on the outside as a recognisable form. Habit is not the same as crystal system. The system describes the underlying lattice; the habit describes the visible shape that lattice happens to produce, which can vary with growth conditions, temperature, and the chemistry of the surrounding fluid.

A vocabulary of shapes

Quartz, which belongs to the trigonal system, has a prismatic habit: long six-sided columns ending in pyramidal points. Pyrite (cubic system) favours a cubic habit, which is why it so often appears as the small brassy cubes found in school collections, though it can also grow as striated pyritohedra (twelve-faced bodies) and as radiating "pyrite suns." Garnet (cubic system) often grows as a dodecahedron, the twelve-sided ball recognisable in almandine and pyrope crystals, or as a twenty-four-faced trapezohedron. Fluorite, also cubic, can grow as straight cubes or as octahedra depending on conditions.

Selenite (a variety of gypsum, monoclinic) is tabular when it grows as the flat blades sold as desk pieces, bladed in slim spear forms, and fibrous in the long parallel needles known as satin spar. Mica grows in sheets thanks to its layered silicate structure. Stibnite forms acicular, needle-like sprays. Pyromorphite grows in botryoidal (grape-cluster) masses. Wavellite shows radial habit, in starbursts of fine fibres. Aragonite often forms dendritic, branching aggregates that resemble coral. When a crystal is too crammed in to express any preferred form it is simply called massive, which is the word for most rose quartz boulders, a chunk of jade, or a piece of obsidian.

Reading habit at the bench

For buyers, habit is a useful identification clue and an aesthetic consideration. A natural quartz point looks the way it does because the mineral grew that way, not because it was carved. The faces of a real quartz prism are slightly uneven, often striated horizontally, and meet at angles that can be measured (the famous law of constancy of angles, observed by Nicolas Steno in 1669, holds that the angles between corresponding faces of a given mineral are constant from specimen to specimen). A polished quartz "point" turned on a lathe gives itself away by being too perfectly symmetrical and by lacking those fine growth striations.

A common misconception is that any geometric stone must be carved. Many are not. Pyrite cubes from Navajún in Spain grow as natural cubes straight from the rock, and are not cut. Fluorite octahedra are usually cleaved from larger crystals, which is a different operation from carving. By contrast, most "obelisks," "towers," and faceted hearts on the market are shaped from massive material, since obsidian, rose quartz, jade, and most jaspers do not grow as well-formed crystals.

Recognising habit is the first step in telling natural specimens from cut and polished pieces, and from glass or resin imitations. It also helps a buyer read what they are looking at: a tabular kunzite tells a different story from a prismatic one, and a botryoidal hematite has a softer presence than the same mineral in sharp rhombohedral crystals.