Pyramidal
Pyramidal crystal forms are, literally, in the shape of pyramids—pyramidal faces predominate in the crystal’s shape. If pyramid faces occur in two directions with the pyramids base-to-base, the habit is dipyramidal.
Acicular
Crystals are said to be acicular when they are thin and needlelike. In general there are few gemstones cut from acicular crystals, but when the needles are parallel and compact, they assume a fibrous form. Satin spar and tiger eye occur in such forms.
Botryoidal
Botryoidal minerals form in globular aggregates, which often resemble bunches of grapes in shape. Hematite, chalcedony, and malachite are minerals that are found in botryoidal form. Malachite, in particular, is often cut and polished across the rounded masses, to reveal bull’s-eye patterns.
Prismatic
Prismatic crystals form long, pencillike shapes in which the length will be several times the diameter. Some prismatic crystals will have very regular, flat, rectangular faces, like aquamarine. In other minerals like tourmaline, the rectangular faces may be curved, to form a cross section like a triangle with curved edges.
Dendritic
In the dendritic crystal habit, aggregates of small crystals form in slender, divergent, somewhat plantlike branches. These are particularly common in copper, silver, and gold. Dendrites of iron and manganese oxide sometimes penetrate chalcedony to form dendritic agate.
Massive
A mineral is said to be massive when it is a mass of tiny crystals that cannot be seen individually. Many gem minerals have their massive counterparts; other gem materials only occur in massive form. Massive minerals tend to be opaque, or at best translucent, and are cut as cabochons or used for carvings.
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