The terms “crystal” and “gem” are often used interchangeably, but they describe two distinct concepts in science and commerce. While a rough diamond is both a crystal and a potential gem, a grain of table salt is a crystal but never a gemstone. The difference lies in whether the material is defined by its internal structure or by standards based on human appraisal and desire. Understanding this distinction requires first looking at the physical science of a crystal.
The Scientific Definition of a Crystal
A crystal is defined purely by its internal, microscopic arrangement of atoms, ions, or molecules. This arrangement must be a highly ordered, repeating, three-dimensional pattern known as a crystal lattice. This structural organization is a fundamental property of the solid material, regardless of its appearance or value.
The smallest repeating unit that builds this lattice is called the unit cell, which, when stacked infinitely, defines the overall structure. Crystallographers classify these internal structures into seven main crystal systems, such as cubic, hexagonal, or monoclinic. The external, geometric shape with flat faces that many minerals exhibit is a macroscopic reflection of this precise internal atomic order.
This scientific definition applies to any solid material with this periodic structure. Many common substances are crystals, such as a snowflake, a grain of sugar, and the quartz in granite, because their constituent particles align in a precise, repeating pattern. The definition is entirely structural and does not factor in the material’s color, size, or usefulness.
The Three Criteria That Define a Gemstone
The term “gemstone,” or simply “gem,” is a human classification applied to select materials that meet three specific criteria: beauty, durability, and rarity. This designation incorporates aesthetic and practical considerations, moving beyond simple structure. A material must possess a pleasing color, brightness, or optical effect to be considered beautiful.
Beauty is measured by factors like color saturation, clarity, and the material’s ability to refract and disperse light, creating “fire” or brilliance. For example, the common mineral corundum becomes the high-value gem ruby or sapphire when trace elements give it a rich color and sufficient clarity.
Durability is the gem’s ability to withstand wear and tear. It is a combination of three properties:
- Hardness, measured on the Mohs scale, which refers to resistance to scratching.
- Toughness, which is the resistance to breaking or chipping.
- Stability, which describes the material’s resistance to chemical damage and changes from heat or light exposure.
Rarity, the final factor, relates to the scarcity of the material in nature, which directly influences its value and desirability. Even a crystal that is beautiful and durable may not be classified as a gem if it is too abundant, like common quartz. The combination of these three factors elevates a mineral specimen to the status of a gemstone, making the classification largely a matter of economic and aesthetic appraisal.
When Crystals Become Gems and the Exceptions
A material becomes a gem when it is a crystal that successfully meets the three criteria of beauty, durability, and rarity. The vast majority of well-known gems, including diamond, ruby, emerald, and amethyst, are mineral crystals. They possess the required ordered atomic structure and satisfy the human-defined standards for lasting aesthetic appeal and scarcity.
Not all crystals are gems, however, because most lack the necessary qualities. For instance, a piece of industrial-grade quartz is a perfect crystal, but its lack of color, clarity, or size prevents it from being classified as a gemstone. This distinction highlights the difference between a scientific classification and a gemological one.
There are important exceptions to the rule that a gem must be a crystal. Some materials are classified as gemstones despite lacking the ordered internal atomic structure; these are called amorphous gems. Examples include opal, a mineraloid, and organic gems such as amber (fossilized tree resin) and pearl (formed by mollusks). Although these materials are not crystalline, their unique beauty and rarity allow them to be included in the gem category.