How are stones classified? Precious, semi-precious, mineral, rock

In everyday language we call almost everything a “stone”, from sand to diamonds and from pearls to meteorites, but science has no class called “stone”. Scientists sort stones by what they are made of and how they formed: minerals, rocks, mineraloids, organic gems and meteorites. Jewellers, laws and ordinary people rank them by value, use and belief instead: “precious”, “semi-precious”, “natural stone”, “birthstone”. The two kinds of ranking do not replace each other: one says what a stone is, the other how people see it.

They are all called stones, but they are not the same thing

Group	What it means	Examples
Mineral	A natural substance with a definite composition and crystal structure	Quartz, diamond
Rock	A natural mixture of minerals	Granite, marble
Mineraloid	A mineral-like natural substance with no crystal structure	Opal, obsidian
Organic gem	Made by living things; not a mineral	Pearl, coral, amber
Meteorite	Rock or metal that fell from space	Chondrite
Everyday name	Not a scientific species but a name from daily life	Sand, gravel, soil, paving stones

On this site these are listed under Everyday names, with the rocks and minerals they are made of. For the difference between minerals and rocks, see Mineral or rock?

Are pearl and coral stones? In the jewellery trade, yes: CIBJO, the World Jewellery Confederation, counts organic materials used in jewellery as gemstones. Scientifically they are not minerals, because living things make them. The gemmologist George F. Kunz wrote in 1913 that the pearl “is not a stone in any sense of the word”, yet it is June’s birthstone today.

How science classifies stones

Minerals: by chemistry

New minerals are approved by the International Mineralogical Association (IMA). Its list updated in September 2026 contains 6,239 valid mineral species, and the number grows every year.

Minerals fall into ten classes by chemical composition, such as elements, sulfides, oxides, carbonates and silicates. This is the Nickel–Strunz system, set up by the German mineralogist Hugo Strunz in 1941, updated with Ernest Nickel in 2001 and supported by the IMA. The older Dana system, first published in 1837, is used mainly in the United States. This site’s classification tree follows Nickel–Strunz.

Many jewellers’ names are colour varieties of a mineral: ruby and sapphire are both corundum, and tanzanite is a variety of zoisite. Science looks at the species, the trade at the variety.

Rocks: by origin

Rocks fall into three families by how they formed: igneous rocks from cooling molten rock, sedimentary rocks from piled-up fragments and deposits, and metamorphic rocks from existing rocks changed by heat and pressure (see The rock cycle).

The names of igneous rocks are set by a subcommission of the International Union of Geological Sciences (IUGS); coarse-grained rocks such as granite, for instance, are named by their proportions of quartz and feldspar. Of the 1,637 terms in the subcommission’s 2002 glossary, only 316 are recommended names. So “how many kinds of rock are there?” has no exact answer.

The trade’s ranking: precious, semi-precious, ornamental

In the traditional split, four stones are “precious”: diamond, ruby, sapphire and emerald. All other gemstones are called “semi-precious”. The list has changed over time: amethyst was once a precious stone, but lost that place after large deposits were found in Brazil in the 19th century.

Gemmologists (gem specialists) find the split misleading, because quality matters more than species. A vivid, clean tanzanite can be worth far more than a pale, cloudy sapphire. That is why CIBJO’s standard is blunt: the term “semi-precious” is misleading and shall not be used. In the same standard, “precious stone” is not a separate class but simply another word for gemstone. What decides quality is explained in What is a gemstone?

What laws and customs say

• US Federal Trade Commission (FTC): its jewellery guides, revised in 2018, still use the words “precious” and “semi-precious” but never define which stones are which; they only call it deceptive to describe artificially produced products as “precious”, “semi-precious”, “natural”, “real” or “genuine”.
• International customs classification (the Harmonized System) still groups gems under “precious stones (other than diamonds) and semi-precious stones”, with rubies, sapphires and emeralds in a subheading of their own.
• Türkiye’s Mining Law No. 3213 makes no such split: in Group V of its article 2, diamond, sapphire, ruby and emerald sit alongside agate, opal, amethyst, turquoise, amber, Oltu stone and meerschaum. Block marble and granite are in Group II, construction sand and gravel in Group I (see Is collecting stones legal?).

Ornamental, natural and worthless stones

• Ornamental stone: CIBJO’s term for gemstones used in decorative objects; it is also used for carving and bead stones such as agate, onyx and jade.
• Natural stone: in the building trade, building and cladding stone such as marble, travertine and granite; in a jewellery shop, “not glass, plastic or a laboratory product”. Neither meaning says anything about value.
• Worthless stone: science has no such class. A single pebble is worth nothing, yet millions of tonnes of sand and gravel are the raw material of concrete and roads (see Is my stone valuable?).

Folk lists: birthstones, zodiac stones, chakras

Birthstones

The modern list of a stone for each month was standardised in August 1912 in Kansas City by a US trade body, the National Association of Jewelers (now Jewelers of America). The list was updated in 1952; tanzanite was added to December in 2002 and spinel to August in 2016. Tanzanite was only discovered in 1967, so this is not an ancient belief but a living marketing tradition. The month-by-month list is on the beliefs page.

Zodiac stones

Zodiac stone lists vary from source to source. Even Kunz’s 1913 book records several month and zodiac lists that disagree: in a Hindu list he quotes, sapphire belongs to July and ruby to August, while in the modern list ruby is July’s stone and sapphire September’s. Different sources today still give different stones for the same sign, and none has a scientific basis.

Chakra colours

Chakras come from Indian traditions, but giving them the colours of the rainbow is a 20th-century Western idea. According to Kurt Leland, writing for the Theosophical Society, single colours in spectral order were first assigned to the chakras in the 1930s, and colours and psychological qualities first appeared together only in 1977. Stone–chakra lists usually follow these colours (red stones for the root chakra, purple ones for the crown) and spread with the New Age movement.

Traditions from Türkiye

• Oltu stone prayer beads: Oltu stone, from the Oltu district of Erzurum, is best known for its prayer beads (tespih). A common belief holds that the beads draw static electricity out of the body, but Oltu stone does not conduct electricity.
• Turquoise and blue stones: the belief that blue stones and beads protect people from harm is old, and studies of the meanings Ottoman Divan poets gave turquoise mention it too. In Anatolia turquoise (firuze) is also worn against the evil eye.
• Agate rings: according to the TDV Encyclopedia of Islam, reports attributed to the Prophet Muhammad, of disputed authenticity, that agate is blessed and that an agate ring keeps poverty away made silver rings set with agate popular.

These meanings are part of culture, but they are not health information; for healing claims, see Healing crystals: what science says.

On this site, both classes side by side

From the street to the diamond: the value ladder

The value ladder on the home page ranks stones in nine steps and explains what drives value at each one:

Step	Name	Examples
1	Everywhere, worthless	River pebbles, sand
2	Raw material by the tonne	Crushed stone, limestone
3	Building and industrial stone	Granite, andesite
4	Fine dimension stone and ore	Marble blocks, boron ore
5	Ornamental stone	Amethyst, agate
6	Fine ornamental and collector stone	Turquoise, Oltu stone
7	Precious stone	Emerald, sapphire
8	Top precious stone	Ruby, diamond
9	Museum-grade rarity	Coloured diamonds, historic jewels

The ladder shows the typical place of a species; even two stones of the same species can differ enormously with quality. It is not investment advice.

The “Popularly” label

Next to its scientific class, every stone page shows how people commonly class the stone: the amethyst page, for example, reads “Popularly: Semi-precious stone”. We use seven popular classes: precious stone, precious metal (gold, silver), semi-precious stone, natural stone (building and decor), ore and raw material, common stone and meteorite. We show “semi-precious” not because it is a sound class, but because people search by that name. Where relevant, the page also gives the birth month, the areas of belief and how far those beliefs are from science.

You can filter the list of all stones by popular class (for example semi-precious stones) or by area of belief, and sort it by conflict with science. Who holds each belief, and what science says about it, is on the beliefs pages.