What Makes a Gemstone? Beauty, Durability, and Rarity

Thousands of minerals exist, but only a few dozen are commonly cut as gemstones. The difference is not chemistry. A gemstone is any material that people choose to cut and wear because it combines beauty with enough durability to survive daily use, and is uncommon enough to feel special. This article looks at each of those qualities and why a stunning mineral specimen can still be a poor gem.

Beauty: Color, Clarity, and Light

The first thing anyone notices in a gem is how it handles light. Several optical properties contribute:

Color comes from the elements in a mineral’s structure or from traces of impurities. Corundum, for example, is colorless when pure, red as ruby when chromium is present, and blue as sapphire with iron and titanium.
Transparency and clarity describe how freely light passes through and how many visible inclusions are present.
Brilliance is the light returned to the eye from inside the stone. It depends partly on the material’s refractive index and largely on how well it is cut.
Dispersion is the splitting of white light into spectral colors, the “fire” that diamond is known for.

Some gems are prized for special optical effects: the shifting play of color in opal, the cat’s-eye line in chrysoberyl, or the six-rayed star in star sapphire, which is caused by aligned needle-like inclusions.

Durability: Hardness, Toughness, and Stability

A gem has to survive being worn, bumped, and cleaned. Gemologists break durability into three parts:

Hardness is resistance to scratching, measured on the Mohs scale. Everyday dust contains quartz at hardness 7, so gems softer than 7 slowly lose their polish. Diamond (10), corundum (9), and topaz (8) resist scratching well.
Toughness is resistance to chipping and breaking. It is separate from hardness. Diamond is extremely hard but can split along its cleavage planes if struck at the wrong angle. Jade, particularly nephrite, is only moderately hard but extremely tough because of its interlocking fibrous structure.
Stability is resistance to heat, light, and chemicals. Opal contains water and can crack if it dries out or is heated. Some kunzite and amethyst fade in strong light. Pearls are damaged by acids, including perspiration and cosmetics.

This is why calcite, fluorite, and many other beautiful minerals stay in display cases rather than jewelry. The hardness test in Reading a Rock explains the Mohs scale in more detail.

Rarity and Tradition

Rarity plays a part in which materials are valued. Fine emerald, ruby, and certain colors of sapphire are scarce in large, clean pieces, while quartz is abundant. Demand also matters. Tanzanite, a blue variety of zoisite found commercially in only one small area of Tanzania, became widely known only after the 1960s.

You will often hear stones divided into “precious” (diamond, ruby, sapphire, emerald) and “semi-precious” (everything else). This is a historical trade convention with no scientific basis, and many gemological organizations discourage it, because some so-called semi-precious stones are rarer than some precious ones.

Gem Species and Varieties

Gemologists classify gems by species, the underlying mineral, and variety, a named color or type within that species. A few examples:

Corundum: ruby (red) and sapphire (every other color).
Beryl: emerald (green), aquamarine (blue), morganite (pink), and heliodor (yellow).
Quartz: amethyst, citrine, and many others, covered in Quartz and Its Many Varieties.
Garnet: actually a group of related minerals, including almandine, pyrope, and grossular.

Knowing the species tells you what to expect in terms of hardness and care, regardless of the trade name a stone is sold under.

Cut Versus Rough

Most gem material looks unremarkable in the rough. Cutting is what reveals its potential. Faceting adds flat, angled surfaces that reflect light back through the top of the stone, while cabochon cutting produces a smooth, domed surface that suits opaque stones and optical effects such as stars and cat’s-eyes. A gem’s appearance therefore depends as much on the cutter’s skill as on the material itself.

What This Site Does Not Do

This article explains how gemstones are defined. It is not a guide to value. Grading and valuing a gem requires laboratory testing and professional judgment, and Steiner Collectables does not appraise, grade, or authenticate stones. If you need to know whether a stone is natural or what it is worth, a qualified gemologist or an accredited gem laboratory is the right resource. For the difference between natural, lab-made, and imitation stones, see Natural, Synthetic, and Simulated Gemstones.