Summary
Crystallography is the study of the formation and structure of crystalline solids. Gemologists deal with many crystalline gems and use descriptive terms to help visualize how these materials developed.
Reading time: 5 min
crystallography - equant quartz with hexagonal pyramidal terminations

These equant quartz crystals have hexagonal pyramidal terminations. These are just some of the crystallographic terms gemologists can use to describe crystalline gem materials. Orange River, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.

Crystal Systems Review

When crystals form, their atoms and molecules lock together in periodic arrays, much like three-dimensional wallpaper patterns. These arrays have various types of symmetry, so gemologists classify them into six major crystal systems:

Some mineralogists consider the trigonal subclass of the hexagonal system as a seventh crystal system.

crystallography - prismatic hexagonal hedyphane

These lustrous, yellow-tan hedyphanes formed as hexagonal, prismatic crystals. However, the terminations or ends of the crystals have pyramidal shapes. Tsumeb mine, Tsumeb, Otjikoto Region, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.

Each crystal system is defined in terms of crystal axes and angles.

  • Crystal axes are imaginary lines in space between the sides of the crystals. They intersect at a common point, and their lengths may be described as equal or unequal to each other.
  • The crystal axes intersect each other at various angles, which further describe the crystal systems.

Terms such as octahedral (8-sided) and dodecahedral or pyritohedral (12-sided) are sometimes used to describe forms characteristic of specific crystal systems.

For more information on crystallography, see our article on crystal systems and mineral habits and our table of gems ordered by crystal system.

Descriptions of Crystal Structures

Crystallography uses additional terms to describe the crystal structures exhibited by various mineral species. The following are some of the most frequently used descriptive terms.

Prismatic

Crystals that form in a prismatic structure have well-developed, elongated, prism-like crystal faces.

crystallography - prismatic cerussites

Prismatic cerussite crystals. The longest crystal measures 2.4 cm in length. Bunker Hill Mine, Kellogg, Shoshone Co., Idaho, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.

Bladed

A bladed crystal has slender and flattened blade-like formations rather than prism-like faces.

crystallography - bladed baryte

This mineral specimen has pastel purple fluorite crystals on white bladed barytes that look like “forests of ridges.” Berbes Mining area, Ribadesella, Asturias, Spain. © Rob Lavinsky, www.iRocks.com. Used with permission.

Acicular

Acicular crystal formations feature slender, possibly tapered, needle-like crystals.

crystallography - acicular pentagonite

This pentagonite specimen looks a teal-blue Christmas tree composed of acicular, transparent crystals. Wagholi quarry, near Poona, Maharashtra, India. © Rob Lavinsky, www.iRocks.com. Used with permission.

Filiform

Filiform crystals are hair-like and extremely fine.

crystallography - filiform millerite

Hair-like, filiform millerite in a quartz geode. Hall’s Gap Road Cut, Lincoln County, Kentucky, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.

Equant

Sometimes referred to as stout crystals, equant crystals have lengths, widths, and breadths roughly equal in size.

crystallography - equant bixbyite

Not the very rare gem known as bixbite or red beryl, this single equant bixbyite crystal perches on a base of topaz and smaller bixbyites. (Bixbite and bixbyite were both named after the mineralogist Maynard Bixby). Solar Wind Claim, Thomas Range, Juab Co., Utah, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.

Pyramidal

Crystals that form pyramidal structures resemble single or double pyramids.

crystallography - pyramidal wulfenite

Pyramidal wulfenite crystals. Onderra Mine, Kaokoveld Plateau, Kunene Region, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.

Tabular

Tabular formations feature a tablet shape with crystals slightly longer than wider.

crystallography - tabular peridot

A tabular peridot crystal perched on a magnetite. Sapat Gali, Naran, Kaghan Valley, Khyber Pakhtunkhwa, Pakistan. © Rob Lavinsky, www.iRocks.com. Used with permission.

Descriptions Based on Aggregation States

Crystallography uses other terms to describe crystals based on their aggregation states. These terms include the following.

Massive

A solid, chunky aggregate without noticeable crystalline shape (although the material is composed of crystals). This term refers to the aggregate’s outward appearance, not its size.

crystallography - ruby and massive scapolite

Ruby in a massive scapolite matrix. Dattaw Hill, Dattaw, Mogok, Myanmar. © Rob Lavinsky, www.iRocks.com. Used with permission.

Botryoidal

A cluster of round, grape-like aggregates.

crystallography - botryoidal apatite

Botryoidal apatite. Astillero Mine, Mapimi, Durango, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.

Compact

A dense, solid aggregate.

crystallography - compact aggregate inesite

Compact cluster of several spherical aggregates of radiating inesite crystals. Wessels Mine, Kalahari manganese fields, Northern Cape Province, South Africa. © Rob Lavinsky, www.iRocks.com. Used with permission.

Cleavable

Denotes a crystalline mass that can be cleaved.

Granular

Comprised of a mass of compact grains.

crystallography - granular hyalite opal

An unusual granular hyalite opal. Erongo Mountain, Usakos and Omaruru Districts, Erongo Region, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.

Stalactitic

Describes aggregates that resemble stalactites.

crystallography - stalactitic smithsonite

This smithsonite and galena specimen features rare, stalactitic smithsonite “fingers.” Touissit Mine, Oujda-Angad Province, Oriental Region, Morocco. © Rob Lavinsky, www.iRocks.com. Used with permission.

Oolitic

Aggregates comprised of masses of spherical grains.

crystallography - oolitic hematite

Oolitic hematite. Photo by GOKLuLe. Licensed under CC By-SA 3.0.

Earthy

Aggregates made of masses of densely packed powder.

Gem Formation and Descriptive Crystallography

A mineral’s growth process and formation environment largely determine its appearance. For example, minerals that form in sedimentary environments tend to be earthy, stalactitic, oolitic, and sometimes massive. On the other hand, igneous minerals tend to be crystalline or massive, sometimes cleavable.

Although these terms are somewhat subjective, they give gemologists a mental image of a mineral’s appearance as it occurs in the Earth.

crystallography - amethyst phantom pyramids

Sometimes during a crystal’s formation, a new layer grows over a transparent crystal. When the growth of the original crystal resumes, it’s left covered with a “shroud.” These inclusions of now indistinct, almost transparent crystals are called phantoms. This amethyst specimen contains pyramidal phantoms. La Sirena Mine, Mun. de Guanajuato, Guanajuato, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.