What is a gemstone? The truth about “precious” and “semi-precious”

A gemstone is a natural material used in jewellery or decorative objects because of its beauty, rarity and durability. Most gemstones are minerals, but some rocks such as lapis lazuli and organic materials such as pearl and amber belong to the group too. The split into “precious” and “semi-precious” stones is historical rather than scientific, and misleading: a stone’s worth depends far more on its colour, clarity, cut, weight, treatments and whether it is natural than on its species.

Three things that make a gem

According to CIBJO, the World Jewellery Confederation, gemstones are inorganic, organic or biogenic materials formed entirely by nature without human interference, used in jewellery thanks to a combination of beauty, rarity and relative durability.

• Beauty: colour, lustre, brilliance or eye-catching effects such as the play of colour in opal.
• Rarity: however beautiful, a stone that is easy to find is worth less.
• Durability: the ability to withstand scratches, knocks, light and chemicals while being worn. How this is measured is explained in The Mohs hardness scale.

“Precious” and “semi-precious”: a historical split

The traditional Western classification, which goes back to the ancient Greeks, divides gems into “precious” and “semi-precious”. Today “precious” usually means four stones: diamond, ruby, sapphire and emerald. Everything else is called “semi-precious”.

Three examples show why this split is misleading:

• Amethyst was once counted among the most precious stones. When large deposits were found in Brazil it became easy to obtain and lost that status. The label depended on supply and fashion, not on the stone itself.
• Spinel was mistaken for ruby for centuries. Even the famous “Black Prince’s Ruby” in the British crown is actually a spinel.
• Quality varies enormously within a single species. A cloudy, cracked ruby carries the “precious” label, while a vividly coloured, clean spinel or tanzanite can be a far more sought-after stone.

That is why CIBJO’s gemstone standard says plainly: the term “semi-precious” is misleading and shall not be used. In the same standard, “precious stones” is not a separate class but simply another name for gemstones.

The 4Cs: colour, clarity, cut, carat

The “4Cs” used to describe diamond quality began as a memory aid that Robert M. Shipley, founder of the Gemological Institute of America (GIA), coined for his students in the 1940s. In 1953 GIA turned these factors into a systematic grading method with its D-to-Z colour scale and its clarity scale.

C	What it describes
Colour	In diamonds, how close the stone is to colourless; in coloured gems, the hue, saturation and vividness of the colour
Clarity	How few internal features (inclusions) and surface blemishes the stone has
Cut	The proportions and how well the facets return light
Carat	Weight: one metric carat is 200 milligrams, or 0.2 grams

The carat used for gemstones is a unit of weight; do not confuse it with the karat that expresses the purity of gold.

There is no single, universally accepted grading system for coloured stones as there is for diamonds, but experts generally regard colour as the most important factor. What determines the value of each stone is described in the “What determines its value?” section of the stone pages on this site.

Treatments: natural, but enhanced

Many natural stones on the market have been treated to improve their appearance or durability. CIBJO defines treatment as artificially changing a gem’s appearance or durability by heating, diffusion, irradiation, filling, coating or other processes.

Treatment	What is done	Common examples
Heating	Improves colour and clarity	Sapphire, ruby, tanzanite; citrine made from heated amethyst
Oil or resin filling	Makes surface-reaching fractures less visible	Emerald
Lead-glass filling	Fills the cavities of heavily fractured material with glass	Low-grade ruby
Irradiation and heating	Gives colour to a colourless stone	Blue topaz
Dyeing	Colours a porous stone	Chalcedony and agate
Stabilisation	Impregnates a porous stone with resin	Turquoise

A treated stone is still a natural stone, but the type and extent of treatment strongly affect its value, and the seller should disclose it. Under the guides of the US Federal Trade Commission (FTC), for example, it is deceptive to sell material filled with lead glass simply as “ruby” or “treated ruby”; it must be clearly disclosed as a product combined with glass.

Synthetics and imitations

• A synthetic stone is made in a laboratory and has essentially the same chemical composition, physical properties and structure as its natural counterpart. According to CIBJO, “synthetic”, “laboratory-created” and “laboratory-grown” mean the same thing. The first synthetic gem was ruby, produced by Auguste Verneuil’s flame-fusion process developed in 1902. Laboratory-grown diamonds are now common too; chemically and physically they are real diamond, but they are not natural diamond.
• An imitation is simply a different material that looks similar: coloured glass, plastic, or cubic zirconia used in place of diamond. Cubic zirconia is a laboratory product and should not be confused with zircon, a natural mineral.

The FTC guides also say that gem names such as “ruby” or “sapphire” may be used for laboratory products and imitations only when immediately preceded, just as conspicuously, by a clear qualifier such as “laboratory-grown” or by the word “imitation”. To learn how to spot imitations, see the Fake gemstones guide.

Organic gems

Some gems are products of living things, and CIBJO counts them as gemstones when they are used in jewellery:

• Pearl: most pearls sold today are cultured pearls grown on farms; natural pearls are very rare.
• Amber: fossilised tree resin; very light.
• Coral: the skeletons of marine animals.
• Oltu stone: a type of jet formed from coalified wood.

Organic gems are usually soft and can be affected by sweat, perfume and heat, so they need careful wear. Why they are not counted as minerals is explained in Mineral or rock?.

Two gems from Türkiye

• Colour-change diaspore (zultanite): diaspore is a mineral that looks greenish in daylight and pinkish under incandescent light. According to GIA, gem-quality diaspore comes almost exclusively from the İlbir Mountains near Milas in Muğla province. In Türkiye it is known as sultanit, and on the international market under trade names such as “Zultanite” and “Csarite”. As its fame grew, imitations followed: in 2016 GIA documented colour-change glass sold as an imitation of this gem.
• Oltu stone: a black, lightweight type of jet mined by hand from narrow galleries in the Oltu district of Erzurum. Its best-known product is the prayer bead string (tespih). In 2012 the name “Oltu Taşı” was registered as a geographical indication by TÜRKPATENT; products bearing the name must be made from raw material mined in Oltu.

For more of Türkiye’s distinctive stones, see Stones of Türkiye.

What to check when buying a gemstone

• Ask whether the stone is natural, synthetic or an imitation, and which treatments it has had; ask for this to be written on the invoice or certificate.
• For a significant purchase, ask for a report from an independent gemmological laboratory. A trade name on a label does not prove what a stone is.
• Look at the stone’s own qualities, not at a “precious” or “semi-precious” label.

If you are wondering whether a stone you own is valuable, Is my stone valuable? can help.