The terms “gem” and “crystal” are often used interchangeably, causing confusion. These classifications belong to different categories: crystal is a precise scientific definition based on atomic arrangement, while gem is a human designation based on aesthetic and commercial value. Understanding whether all gems are crystals requires separating these two classification systems to examine their overlap and divergence.
The Defining Structure of a Crystal
A crystal is defined by a highly ordered, repeating internal arrangement of atoms, ions, or molecules. This structure is known as a crystal lattice, which extends uniformly in three-dimensional space. The lattice is built from a repeating block called the unit cell, which dictates the material’s fundamental properties. A solid must possess this long-range, periodic order to be scientifically classified as crystalline.
The internal atomic order is the sole defining characteristic, regardless of the material’s outward appearance. While a well-formed crystal may exhibit flat, geometric faces, this external shape is merely a macroscopic reflection of the perfect internal symmetry. Solids that lack this repeating pattern, like glass, are classified as amorphous, meaning they are “without form” on an atomic scale. The scientific study of this specific atomic arrangement is known as crystallography.
The Human Criteria for a Gemstone
A material earns the designation of a gemstone not through scientific structure, but through a human, commercial, and aesthetic process. This classification is based on three primary factors: beauty, durability, and rarity.
Beauty encompasses qualities like color, luster, transparency, and the way the material interacts with light, often referred to as brilliance. These visual characteristics make the material desirable for adornment.
Durability is measured by a material’s resistance to scratching, chipping, and breaking, ensuring it can withstand regular wear. Gemologists often use the Mohs scale of mineral hardness to assess resistance to scratching.
Rarity contributes directly to the material’s value, as a limited supply of a beautiful and durable material increases its commercial desirability. These human-determined criteria are applied to materials after they have been cut and polished for use in jewelry.
The Overlap: When Gems Are Crystalline
The vast majority of the world’s most recognized and valuable gemstones are, in fact, crystals. These materials satisfy both the strict scientific requirement of having a crystalline lattice and the human criteria of beauty, durability, and rarity. Diamond, the hardest known material, is a prime example, consisting of a cubic crystalline lattice of carbon atoms.
Ruby and sapphire are both varieties of the mineral corundum, which forms in the hexagonal crystal system. Trace amounts of impurities, such as chromium for ruby and iron/titanium for sapphire, are incorporated into the corundum lattice, creating their vivid colors.
Quartz, a widely used gem, also forms a hexagonal crystal structure, with varieties like amethyst and citrine demonstrating the beauty derived from this internal order. These common examples show that most traditional gems are simply crystalline minerals that meet the high standards set by people.
Materials Used as Gems That Are Not Crystals
Not all gems are crystals, as some materials meet the human criteria for a gemstone without possessing a crystalline structure. These exceptions fall into two main categories: amorphous mineraloids and biogenic materials. Amorphous materials lack the repeating atomic lattice that defines a crystal. Obsidian, a form of volcanic glass, is a naturally occurring amorphous gem that forms when lava cools too quickly for a crystal structure to develop.
Opal is another famous amorphous example, classified as a mineraloid because its hydrated silica composition lacks a strict lattice. It has a highly organized structure of silica spheres that creates its unique play-of-color, but it is not crystalline.
Biogenic gems originate from living organisms and are typically amorphous or a combination of organic and mineral components. Amber, which is fossilized tree resin, is an amorphous organic material cut and polished for jewelry. Pearl, created by mollusks, is soft and porous, lacking a true crystalline arrangement.