Descriptive Crystallography for Gemologists
Table of Contents
Crystal Systems Review
When crystals form, their atoms and molecules lock together in period arrays, much like three-dimensional wallpaper patterns. These arrays have various types of symmetry. Gemologists classify them into six major crystal systems:
Some mineralogists consider the trigonal subclass of the hexagonal system as a seventh crystal system.
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. Their lengths may be described as equal or unequal to each other.
- The crystal axes intersect each other at various angles. The angles further describe the crystal systems.
Descriptions of Crystal Structures
Crystallography uses additional descriptive terms to explain the crystal structures exhibited by various mineral species. These include the following.
Crystals that form in a prismatic structure have well-developed, elongated, prism-like crystal faces.
A bladed crystal has slender and flattened blade-like formations rather than prism-like faces.
Acicular crystal formations feature slender, possibly tapered, needle-like crystals.
Filiform crystals are hair-like and extremely fine.
Equant crystals have lengths, widths, and breadths roughly equal in size. Sometimes, equant crystals are referred to as stout crystals.
Crystals that form pyramidal structures resemble single or double pyramids.
Tabular formations feature a tablet shape with crystals slightly longer than wider.
You may encounter other terms for describing a mineral’s appearance, such as octahedral (8-sided) and pyritohedral (12-sided). However, these terms are typically used in conjunction with a specific crystal system.
Descriptions Based on Aggregation States
Crystallography uses more terms to describe crystals based on their aggregation states. These terms include the following.
A solid, chunky aggregate.
A dense, solid aggregate.
Denotes a crystalline mass that can be cleaved.
Comprised of a mass of compact grains.
Describes aggregates that resemble stalactites.
Aggregates comprised of masses of spherical grains.
Aggregates made of masses of densely packed powder.
Gem Formation and Descriptive Crystallography
A mineral’s growth process and formation environment largely determines 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 serve to give gemologists a mental image of a mineral’s appearance as it occurs in the Earth.