A gemstone is a piece of mineral, rock, or organic material that is cut and polished for use in jewelry or decoration. What separates a gemstone from an ordinary rock is a combination of three qualities: beauty, durability, and rarity. Most gemstones are minerals that formed deep inside the Earth over millions of years, though a few come from living organisms instead.
What Gemstones Are Made Of
The vast majority of gemstones are minerals with specific chemical structures. Diamond is pure carbon that crystallized under extreme pressure. Ruby and sapphire are both varieties of the same mineral, corundum, which is essentially aluminum oxide. The only difference between them is the trace element that gives each its color. Emerald is a form of the mineral beryl, built from beryllium, aluminum, silicon, and oxygen. Quartz, garnet, topaz, and tourmaline are all distinct minerals with their own chemical recipes.
A smaller group of gemstones are not minerals at all. They come from biological sources. Pearl forms inside a mollusk when the animal coats an irritant with layer after layer of a smooth substance called nacre. Amber is fossilized tree resin, sometimes millions of years old, occasionally preserving ancient insects inside. Coral is built by tiny marine animals that extract calcium carbonate from seawater to construct their skeletons. Jet is a form of compact, polished coal. These four organic gemstones are softer and more delicate than most mineral gems, but they’ve been prized for centuries.
How Gemstones Form in the Earth
Gemstones occur across nearly every major type of geological environment. Most are found in igneous rocks (formed from cooling magma) or in gravel beds where erosion has concentrated heavier, more durable minerals over time. Metamorphic rocks, which have been transformed by intense heat and pressure, also produce gems. Diamonds, for example, form at depths of at least 100 kilometers (about 60 miles) below the surface and are carried upward through narrow, pipe-shaped rock formations called kimberlites.
Many colored gemstones crystallize in pegmatites, which are bodies of coarse-grained igneous rock that form as pockets of mineral-rich fluid cool slowly underground. The slow cooling allows large, well-formed crystals to develop. Once exposed by erosion, these crystals can wash into streams and rivers, settling into gravel deposits called placers. Some of the world’s most productive gem-mining areas are placer deposits rather than hard-rock mines.
Hardness and Durability
A gemstone’s durability is measured partly by its hardness, or resistance to scratching. The standard reference is the Mohs scale, a ranking from 1 (talc, the softest) to 10 (diamond, the hardest). Any mineral can scratch one that ranks lower on the scale, but not one that ranks higher.
Here’s where the most common gemstones fall:
- Diamond: 10
- Ruby and sapphire (corundum): 9
- Chrysoberyl (alexandrite): 8.5
- Topaz and spinel: 8
- Emerald and aquamarine (beryl): 7.5 to 8
- Tourmaline: 7 to 7.5
- Quartz (amethyst, citrine): 7
- Peridot: 7
- Garnet: 6.5 to 7.5
- Opal: 5.5 to 6.5
- Turquoise: 5 to 6
- Lapis lazuli: 5 to 5.5
A hardness of 7 or above is generally considered durable enough for everyday ring wear, since quartz dust (hardness 7) is one of the most common abrasives in household dust. Gems below 7 scratch more easily and are better suited for earrings, pendants, or pieces worn occasionally.
Precious vs. Semi-Precious
Traditionally, gemstones have been divided into two categories. Precious gemstones possess all three key qualities: beauty, durability, and rarity. This group includes diamond, ruby, sapphire, emerald, and opal. Everything else, from amethyst to turquoise, has typically been labeled semi-precious, meaning it has one or two of those qualities but not all three.
In practice, this distinction can be misleading. A fine Paraíba tourmaline or alexandrite can sell for far more per carat than a low-quality sapphire. Many gem professionals now avoid the term “semi-precious” because it suggests lower value across the board, which simply isn’t the case. The quality of an individual stone matters more than its category.
What Determines a Gemstone’s Value
For colored gemstones, color is the single most important factor, accounting for roughly 60% of a stone’s value according to industry estimates from the Gemological Institute of America. Color is evaluated on three dimensions: hue (the actual color, like red or blue), saturation (how vivid or rich that color appears), and tone (how light or dark it is). Uneven color distribution within a stone, called color zoning, lowers its worth.
The remaining value comes from several other factors. Country of origin can influence about 15% of the price, and for certain stones this premium is even higher. A sapphire from Kashmir or a ruby from Burma commands significantly more than a chemically identical stone from a less storied source. Size and cut quality each account for around 10% of value. Clarity, the stone’s freedom from visible flaws, also plays a role, though its importance varies by gem type. Some gems, like emeralds, are expected to have inclusions, while others, like aquamarine, are expected to be clean.
Lab-Grown vs. Natural Gemstones
Synthetic or lab-grown gemstones are chemically and physically identical to their natural counterparts. A lab-grown ruby is real corundum with the same crystal structure and hardness as a mined ruby. The difference is origin: one formed in the Earth over millions of years, the other was created in a controlled environment over weeks or months.
The most reliable way to tell them apart is by examining inclusions under magnification. Natural gems almost always contain tiny internal features, such as trapped minerals, growth lines, or fluid pockets, that reflect their geological history. Synthetics tend to be remarkably clean. If a gemstone looks flawless under 10x magnification, that near-perfect clarity is itself a clue that it may be lab-grown. Synthetics also tend to display exceptionally vivid, saturated color, since they’re engineered to mimic the best examples of each gem type. A stone with only modest color saturation is more likely natural.
Some of the Rarest Gemstones
Diamond gets the most attention, but several gemstones are actually rarer. Tanzanite occurs in only a single small mining area in Tanzania, making it one of the most geographically limited gems on Earth. Alexandrite, first discovered in Russia’s Ural Mountains in 1830, is famous for appearing green in daylight and red under incandescent light. Later finds in Brazil expanded supply slightly, but it remains exceptionally scarce. Paraíba tourmaline, known for its electric neon blue-green color caused by traces of copper, was first found in the Brazilian state of Paraíba and has since turned up in Mozambique and Nigeria, though quantities are still tiny.
Fine Burmese rubies are among the most valuable gems per carat on the market. Jadeite, the rarer of the two minerals called jade, has been treasured in Chinese and Mesoamerican cultures for thousands of years. At the highest quality levels, these stones can sell for more per carat than diamonds of comparable size.