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A Guide to the World's Famous Gemstones

See what mineral each gemstone is made of, why it has the color it does, how hard it is, and what to know before you buy one. The ten stones below cover the classic precious gems plus several popular ones that are easy to confuse.

Diamond: The Hardest Gem, Made of Pure Carbon

Diamond forms when carbon atoms bond tightly in a cubic crystal structure, crystallizing deep in the mantle under intense heat and pressure before volcanic eruptions carry it toward the surface. Pure diamond is colorless, but trace nitrogen can tint it yellow and trace boron can tint it blue. It sits at 10 on the Mohs scale, the top of the scale. Historically India was the major source, followed by South Africa; in recent decades large producers have included Russia, Botswana, and Canada, though rankings shift. Lab-grown diamonds have the same chemistry and structure as mined ones, so the difference is origin, not material.

Ruby: Corundum Colored Red by Chromium

Ruby is a variety of corundum (aluminum oxide) that owes its red color to trace chromium. Corundum ranks 9 on the Mohs scale, making ruby very durable for everyday jewelry. Myanmar's Mogok region has long been prized for fine rubies, with Thailand, Sri Lanka, and Mozambique also significant sources. Heat treatment to improve color and clarity is very common and generally accepted in the trade when disclosed. Any corundum that is not red is classified as sapphire instead, and the boundary between a pinkish ruby and a pink sapphire is set by trade convention, not by a single chemical line.

Sapphire: Every Color of Corundum Except Red

Sapphire is the same corundum mineral as ruby, but iron and titanium give the classic blue color. Any red corundum is called ruby; every other color, including yellow, pink, orange, and green, is called a fancy sapphire. Kashmir, Sri Lanka, Madagascar, and Australia are notable sources. Like ruby it measures 9 on the Mohs scale, and heat treatment is routine. The rare pinkish-orange padparadscha is a named variety whose exact color boundaries are debated even among gem laboratories.

Emerald: Beryl Colored Green by Chromium and Vanadium

Emerald is beryl with trace chromium or vanadium giving it a deep green color. It ranks about 7.5 to 8 on the Mohs scale, but natural fractures inside most emeralds make it more brittle than the number suggests. Fracture filling with oil or resin is a standard practice, so the treatment level matters to value. Colombia's Muzo area has historically been a top source, with Zambia and Brazil also major producers. Ancient Egypt is said to have mined emeralds, and Cleopatra is traditionally linked to the stone, though the historical details are uncertain.

Pearl: An Organic Gem Grown Inside a Shell

Unlike other gems, pearl is not a mineral. It forms when a mollusk such as an oyster or mussel deposits layer upon layer of nacre, made mostly of calcium carbonate, around an irritant. Natural pearls are extremely rare, so nearly all pearls sold today are cultured, grown by deliberately inserting a bead or tissue graft. Kokichi Mikimoto of Japan is widely credited with commercializing pearl culturing in the early 20th century. Pearl is soft, roughly 2.5 to 4.5 on the Mohs scale, and is damaged by acids, perfume, and rough storage.

Opal: A Gem That Diffracts Light Into a Rainbow

Opal is hydrated silica. Its play of color comes from tiny, orderly silica spheres that diffract light, so the colors shift as you turn the stone. Opal contains water, from a few percent up to a considerably higher share in some specimens, and can crack if it dries out suddenly or is heated. It ranks about 5.5 to 6.5 on the Mohs scale. Australia has long produced the large majority of the world's precious opal, and black opal, with vivid color against a dark body tone, is among the most valued types.

Amethyst: Purple Quartz Colored by Iron

Amethyst is quartz that turns purple from trace iron combined with natural radiation in the ground. Quartz is 7 on the Mohs scale. Brazil, Uruguay, and Zambia are major sources. Heating amethyst can change it to yellow or orange, and such stones are sold as citrine, which is why a lot of commercial citrine began as amethyst. The name comes from a Greek word meaning not intoxicated, linked to an old belief that the stone protected against drunkenness. It is listed as the February birthstone in the commonly used modern lists.

Turquoise: A Sky-Blue Gem Formed by Copper and Aluminum

Turquoise is a hydrated phosphate of copper and aluminum, known for its opaque sky-blue to blue-green color. Despite the name, Turkey is not a main source. The name comes from the French for Turkish stone, reflecting the trade route through Turkey by which it reached Europe. It ranks about 5 to 6 on the Mohs scale and is porous, so skin oils, cosmetics, and cleaning products can change its color. Much of the turquoise on the market is stabilized with resin to improve hardness. It has long been prized in ancient Persian and Egyptian jewelry and by Native American cultures such as the Navajo and Zuni.

Garnet: A Silicate Gem Family With Many Colors

Garnet is not a single mineral but a group of closely related silicate minerals, including almandine, pyrope, spessartine, grossular, and andradite. The name is generally linked to the Latin word for pomegranate, because small red crystals resemble the fruit's seeds. Deep red is the best-known color, but garnet also occurs in orange, yellow, and vivid green, as in tsavorite. Hardness varies by type, roughly 6.5 to 7.5. Garnet is the January birthstone in the common modern lists.

Topaz: A Silicate Gem Ranging From Colorless to Golden

Topaz is a silicate mineral containing aluminum and fluorine, usually colorless or pale in its natural state. It is 8 on the Mohs scale, but it has a direction along which it can split cleanly, so a hard knock can chip it. The vivid blue topaz commonly sold today is typically colorless topaz that has been irradiated and heat-treated, while natural deep golden-orange imperial topaz is valued far higher. Topaz is listed among the November birthstones, alongside citrine.

Hardness: How the Mohs Scale Compares These Stones

The Mohs scale ranks scratch resistance from 1 to 10, and it is a ranking rather than a measured ratio. Diamond is 10, ruby and sapphire 9, topaz 8, emerald about 7.5 to 8, amethyst 7, garnet about 6.5 to 7.5, opal about 5.5 to 6.5, turquoise about 5 to 6, and pearl about 2.5 to 4.5. A stone that is hard can still be brittle, so toughness and cleavage also matter for rings that take daily knocks.

Treatments, Lab-Grown Stones, and Disclosure

Many gemstones are enhanced: heat for ruby and sapphire, oil or resin for emerald, stabilization for turquoise, irradiation for blue topaz. Lab-grown stones have the same composition as natural ones and are legitimate when sold as such. Simulants such as cubic zirconia only imitate appearance. Reputable sellers disclose treatments and origin; an independent laboratory report is a reasonable step for high-value purchases.

Why color varies so much within one mineral

Many gem colors come from tiny amounts of other elements, not from the main mineral. Pure corundum is colorless; add a little chromium and it turns red, add iron and titanium and it turns blue. Beryl works the same way, with chromium or vanadium making emerald green while other impurities give aquamarine or morganite. This is why a mineral name and a gem name do not always match: ruby and sapphire are both corundum, and emerald and aquamarine are both beryl. Color can also come from structural features, like the layered silica spheres in opal or radiation-induced defects in amethyst.

What to check before buying a stone

Ask whether the stone is natural or lab-grown, what treatments it has had, and where it came from if that is claimed. For colored stones, color is usually the main value driver, followed by clarity and cut; for diamond, the widely used framework is color, clarity, cut, and carat weight. One carat is a unit of mass equal to 200 milligrams, so a carat figure says nothing about the size of the stone on the finger, because density differs between gems. Prices of gemstones change and vary by market, so this guide gives no price figures. This is general information, not appraisal or investment advice.

Frequently Asked Questions

Why are ruby and sapphire classified as the same mineral?

Both belong to corundum, an aluminum oxide mineral. The color depends on trace elements: red corundum is called ruby, and every other color, including blue, yellow, and pink, is classified as sapphire by long-standing trade convention.

Why is pearl different in character from other gemstones?

Most gemstones are minerals formed underground, but pearl is an organic gem created biologically inside a mollusk. It is much softer than mineral gems and reacts to acids and perfumes, so it is usually treated as a separate category.

Is a lab-grown diamond a real diamond?

Chemically and structurally, yes: it is carbon in the same crystal arrangement. What differs is the origin, which affects how it is described and usually its price. Sellers should label it clearly as lab-grown.

Which of these gems is best for an everyday ring?

Hardness and toughness matter. Diamond, ruby, and sapphire are the most scratch resistant. Emerald, opal, turquoise, and pearl are softer or more fragile and tend to suit pieces worn with more care, such as earrings or pendants. This is general guidance, and a jeweler can advise on a specific setting.

Does a treated gemstone have less value?

Not automatically. Heat treatment of ruby and sapphire is common and widely accepted when disclosed, while untreated stones of top quality are rarer and often priced higher. What matters is that the treatment is disclosed to you.