Crystal Habits and Forms of Minerals and Gems

Acicular crystals have a needle-like shape that tapers to a point or a blunt termination. Many acicular crystals can be clustered to produce fan-shaped or radially-shaped aggregates. The name acicular should be used when the length of an individual crystal is much greater than its width or diameter. Mineral examples include rutile, natrolite, millerite, and gypsum. This geode found in Kentucky contains thin needle-like crystals of millerite. This specimen is also a good example of the geodic habit. Specimen and photo by Arkenstone / www.iRocks.com.

Banded minerals have narrow layers or bands of different color and/or texture. These may be a response to changes in the composition of the growth liquid, the sedimentary process, or other conditions. Mineral examples: quartz (agate), malachite, rhodochrosite, and fluorite. The photo above shows rhodochrosite cabochons that display a banded habit. In one of the cabochons, the banded habit is actually an internal feature of a stalactitic habit. The cabochons were cut from material mined in Argentina, and the cabochon on the left is about two centimeters in length.

Bladed crystals are elongated. They are much longer than they are wide, and their width exceeds their depth. They are shaped like a straight sword or knife blade. Their ends sometimes taper to a point. They might exist as single crystals, a cluster of many parallel crystals, or radiating clusters of crystals. Mineral examples: kyanite, actinolite, and stibnite. These blue crystals of kyanite have a bladed habit. Kyanite crystals are interesting because they have a hardness of 4.5 to 5 parallel to the length of their blades, and a hardness of 6.5 to 7 across the width of their blades. This specimen is approximately seven centimeters across. Image by Aelwyn, used here under a Creative Commons License.

Columnar crystals are long prisms with enough width that the name acicular (or needle-like) does not apply. A single "column" might contain multiple parallel crystals. Mineral examples: calcite, tourmaline, and gypsum. These enormous crystals of selenite gypsum have a columnar habit. They are in the "Cave of the Crystals" cavern, Chihuahua, Mexico (a person in the lower-right quadrant of the photo serves as scale). These are some of the largest well-formed crystals in the world. The photo is a Wikimedia OTRS image by Alexander Van Driessche.

Dendritic crystals form a branching pattern, much like the branches of a tree, the veins in a leaf, or the branching pattern of streams in a drainage basin. Mineral examples: copper, pyrolusite, and other manganese oxide minerals. These crystals of pyrolusite formed on a bedding surface of a piece of lithographic limestone collected near Solnhofen, Germany. The image is a public domain photograph created by Aram Dulyan at the Natural History Museum of London.

Doubly terminated is a name used for a prismatic crystal that has a natural termination on both ends. Normally, crystals have a termination on one end - because the other end of the crystal was attached to the wall of a geode, the roof of a cavern, or a surface that it was growing on. The doubly terminated crystals shown in the photo above are composed of quartz and are known as "Herkimer diamonds" (a misnomer). They are found in cavities of the Little Falls Dolostone in Herkimer County, New York. A few mines where they can be found have become tourist attractions for mineral collectors and treasure hunters who want to find them.

Fibrous is a habit name used when minerals occur in very fine fiber-like crystals. They are often so fine that they look like fine hair. The habit also includes aggregates made up of a large number of parallel or radial fibers. Mineral examples: actinolite, chrysotile, serpentine, and tremolite. The actinolite crystals on this rock have a fibrous habit. Because of their fibrous shape (a roughly 1:20 aspect ratio) and properties, fibrous crystals of actinolite are regulated as asbestos. This photo is a Creative Commons image published by Didier Descouens.

Geodic is a habit in which mineral aggregates form a rounded or oblate mass by crystallization on the inside walls of a cavity. Concentric bands or layers of mineral crystals subsequently develop, gradually infilling the cavity without infilling it completely and with a crystal-lined central void. The specimen in the photo is a geode formed by the precipitation of banded agate to form the external wall and initial layers. The center of the geode is lined with quartz crystals. Image copyright iStockphoto / WojciechMT.

Hopper crystals are partially formed crystals that have experienced more rapid growth on their outer edges than in the center of the crystal. This causes them to be well-developed on the outer edges but less developed or "hollow" in the center. Halite (shown above) is one of the best-known examples of a mineral that sometimes displays the hopper crystal habit. Other mineral examples: galena and ice.

Nodular is the name of a habit in which mineral crystals grow to form rounded or bulbous structures. The crystals are usually arranged in a radial structure within the nodule, even though the nodules may exhibit concentric banding. In the concentric banding, each layer is composed of crystals growing up and outward from the layer immediately below. Mineral examples: quartz (agate), azurite, hematite, realgar, and variscite. A nodule of variscite (bright green), crandallite (canary yellow), wardite (gray) and montgomeryite (dark green). The nodule is approximately eleven centimeters across and was found in Utah County, Utah. Specimen and photo by Arkenstone / www.iRocks.com.

Oolitic minerals occur in crystalline aggregates that are rounded and less than about four millimeters in size. Oolites form by chemical precipitation from a solution. Similar to pisolitic, but oolites are much smaller than pisolites. Mineral examples: hematite and calcite. A specimen of oolitic iron ore approximately ten centimeters across. The largest oolites are a few millimeters across.

Prismatic is a habit name for minerals that form in elongated crystals with opposite faces normally parallel to one another. The crystals are often striated along their length (as in tourmaline) or across their width (as in quartz). Mineral examples: tourmaline, quartz, beryl, hornblende, augite, diopside, and topaz. Shown in the photo above are prismatic crystals of colorful tourmaline from Afghanistan with striations parallel to their long axis. The largest specimen in this photo is about three centimeters in length.

Rosettes are clusters of tabular crystals in a radial arrangement that have an external geometry that resembles a rose or flower. Barite and gypsum sometimes form crystals of this shape in sand to produce a rosette with a sandy appearance. Mineral examples: barite, gypsum, pyrite, and marcasite. The photo shows a "barite rose" which formed when a cluster of barite crystals grew in sand, incorporating many of the sand grains within each crystal. Specimen and photo by Arkenstone / www.iRocks.com.

Striations are fine, slightly indented lines that are present on the faces of some crystals. They always parallel a crystallographic axis and one of the edges of that crystal face. Mineral examples: pyrite, tourmaline, quartz, feldspar, euclase, and topaz. The photo shows a crystal of blue euclase with striations on its faces that parallel the long axis of the crystal. This specimen is also a good example of the prismatic crystal habit. Specimen and photo by Arkenstone / www.iRocks.com.