Chemical Composition of Gemstones: Diamonds, Sapphires, Rubies, Emeralds and Opals

What is the chemical composition of gemstones?

Every gemstone has a definite recipe. Gemologists describe it in two parts: the chemical composition, meaning which elements are present and in what ratio, and the crystal structure, meaning how those atoms are stacked in a repeating pattern. Change either one and you have a different stone.

That is why a gem's chemical formula matters more than it first seems. It explains colour, hardness, how light bends through the stone, and how it should be set, cleaned and stored. When we identify coloured gemstones at the bench, every test we run is really confirming the composition and structure. Any unfamiliar words on this page are also explained in our jewellery glossary.

Four ideas come up again and again:

• Major elements: the building blocks written in the formula, such as aluminium and oxygen in sapphire.
• Trace elements: tiny impurities, often well under 1% by weight, that cause most gem colour.
• Crystal system: the shape of the repeating unit (cubic, hexagonal, trigonal and so on). A stone with no regular structure, like opal, is called amorphous.
• Species and variety: the species is the mineral (corundum); the variety is a colour type within it (ruby, blue sapphire).

What are diamonds, sapphires, rubies, emeralds and opals made of?

The table below covers the gems we are asked about most. Mohs hardness is a 1–10 scale of scratch resistance. Refractive index (RI) measures how strongly a stone bends light; a higher number usually means more brilliance, and it is one of the first readings a gemologist takes.

Notice that the five best-known gems belong to just four species, and two of those species (corundum and beryl) supply most of the colours. Silicon and oxygen, the two most common elements in the Earth's crust, appear in emerald, opal, quartz, garnet, peridot, topaz, tanzanite and zircon. Diamond is the outlier: one element, nothing else.

What is diamond made of, and why does pure carbon matter?

Diamond is pure carbon. Each carbon atom is locked to four neighbours by strong covalent bonds (bonds where atoms share electrons) in a tight three-dimensional framework. That framework is why diamond reaches 10 on the Mohs scale and has a high refractive index of 2.417 and strong dispersion, which together give it its brightness and fire.

Graphite, the grey material in pencils, is also pure carbon. The difference is structure: graphite's atoms sit in flat sheets that slide over each other. Two minerals with the same formula but different structures are called polymorphs, and diamond and graphite are the classic example.

Diamond is not perfectly pure in practice. Gemologists sort diamonds into types by their trace elements:

• Type Ia: nitrogen in clusters. The large majority of natural diamonds; higher nitrogen adds a faint yellow tint.
• Type Ib: scattered single nitrogen atoms, giving strong yellow to orange. Rare in nature, common in HPHT lab-grown diamonds before treatment.
• Type IIa: almost no nitrogen. Rare in nature but typical of many laboratory-grown diamonds.
• Type IIb: contains boron, which turns the stone blue and lets it conduct electricity.

Pink diamonds, including those from the former Argyle mine in Western Australia, are an exception: their colour comes mainly from distortion of the crystal lattice rather than a trace element. Despite its hardness, diamond can burn in air at high temperatures (roughly 700–900°C), which is why jewellers protect diamonds when using a torch. If you are weighing up a softer coloured stone instead, see how emerald compares with diamond.

How can ruby and sapphire be the same mineral?

Ruby and sapphire are both corundum, crystalline aluminium oxide (Al₂O₃). Pure corundum is colourless, sold as white sapphire. A trace of chromium turns it red, and red corundum is called ruby. Every other colour is sapphire.

• Blue sapphire: iron and titanium together. Light shifts an electron between the two, absorbing red and yellow light and leaving blue.
• Yellow and green sapphire: iron on its own, sometimes with colour centres (tiny defects in the crystal that absorb light).
• Pink sapphire: less chromium than ruby. Where pink ends and ruby begins is a matter of lab and trade convention.
• Padparadscha: a pinkish-orange mix, usually chromium with iron or colour centres.
• Parti sapphire: zones of different colour in one stone, common in Australian material.

Australia is one of the world's historic sapphire sources, mainly the New England fields of northern New South Wales around Inverell and Glen Innes, and the Central Queensland gemfields around Anakie, Rubyvale and Sapphire. These sapphires formed in iron-rich conditions linked to volcanic basalts, so many are deep blue, sometimes dark, and green, yellow and parti colours are common. Our sapphire gemstone guide covers colour, origin and treatment in depth.

Corundum's chemistry also makes it unusually durable. It is 9 on the Mohs scale and has no true cleavage (a plane where a crystal splits easily), so its toughness is rated excellent. That is why many people shortlist it for daily wear; compare the two hardest options in diamond vs sapphire engagement rings, or see designs in our sapphire engagement rings guide. Heat treatment is very common in corundum: it changes how iron and titanium sit in the crystal and must always be disclosed.

What is the chemical formula of emerald, and what makes it green?

Emerald is green beryl, a beryllium aluminium silicate with the formula Be₃Al₂Si₆O₁₈. Its silicon and oxygen atoms form six-sided rings stacked into channels, which is why beryl crystals grow as hexagonal prisms. Those channels can hold small amounts of water and alkali elements such as sodium and caesium, which helps labs work out where a stone came from.

The green comes from chromium, vanadium or both, replacing a small amount of aluminium. Some labs call vanadium-coloured green beryl emerald only when the colour is strong enough; paler stones may be reported as green beryl. Other trace elements give beryl's other varieties:

• Aquamarine: iron, giving blue to blue-green.
• Morganite: manganese, giving pink to peach.
• Heliodor: iron in a different state, giving yellow.
• Goshenite: almost no trace elements, so colourless.

Emerald is 7.5–8 on the Mohs scale, harder than quartz, yet its toughness is only poor to fair. The reason is growth, not chemistry alone: most emeralds contain fractures and inclusions, called the jardin (French for garden). Filling those fractures with oil or resin is standard and should be disclosed. Our page on emerald as a gemstone explains grading and treatments, and emerald engagement rings covers protective designs.

Morganite shares emerald's formula but usually has far fewer fractures, so it behaves differently on the hand. See morganite's pros and cons for engagement rings.

What is opal made of, and why does it contain water?

Opal is hydrated silica, written SiO₂·nH₂O, where n means the amount of water varies. Most opal holds about 3–10% water by weight. Because it has no regular crystal structure, opal is classed as a mineraloid rather than a true mineral.

Look at precious opal under an electron microscope and you see tiny spheres of silica, roughly 150–300 nanometres across, packed in orderly rows. Light diffracts (splits) as it passes between them, producing play-of-colour. Smaller spheres give mainly blues and greens; larger spheres are needed for reds, which is one reason red-dominant opal is rarer. Common opal, or potch, has the same chemistry but disordered spheres, so it shows no play-of-colour.

Opal is Australia's national gemstone, and the type names mostly describe body tone and host rock rather than chemistry:

• Black opal: a dark body tone behind the colour, found mainly at Lightning Ridge, NSW.
• White opal: a light, milky body tone, classically from Coober Pedy, South Australia.
• Crystal opal: transparent to translucent, so colour seems to sit inside the stone.
• Boulder opal: thin seams of opal in ironstone from western Queensland, usually cut with the host rock left on the back.
• Fire opal: transparent yellow to red body colour from traces of iron, best known from Mexico.

Water is opal's weak point. If a stone loses water, for example under strong heat or very dry storage, it can develop crazing, a network of fine cracks. Some Ethiopian opal is hydrophane, meaning it soaks up water and can temporarily change appearance; most Australian sedimentary opal is more stable. The differences are set out in Australian and Ethiopian opal compared.

What causes gemstone colour?

Gemologists split coloured stones into two groups. Idiochromatic gems are coloured by an element in their own formula, so they come in a narrow colour range: peridot is always green because iron is part of olivine. Allochromatic gems are colourless when pure and take colour from trace elements, which is why corundum and beryl come in so many shades.

Not all colour is chemical. Opal's play-of-colour is a structural effect of its silica spheres, pink diamond colour comes from lattice distortion, and amethyst's purple needs both iron and natural radiation to form colour centres. The same element can also act very differently: chromium makes ruby red but emerald green, because the atoms around it change how it absorbs light.

How does the chemical composition of gemstones affect durability in a ring?

Durability has three parts. Hardness is resistance to scratching. Toughness is resistance to chipping and breaking, and depends on bonding and cleavage. Stability is resistance to heat, chemicals and light. Household dust contains quartz at Mohs 7, so stones softer than 7 slowly lose their polish in everyday wear.

Chemistry explains the surprises. Diamond is the hardest gem yet can cleave if struck hard on the right angle. Corundum is a step softer but tougher, with no easy plane of weakness. Opal's water and amorphous structure make it softer and sensitive to heat, so read opal engagement ring wearability and care before choosing one for daily wear. If you are torn between two coloured stones, emerald versus sapphire for an engagement ring compares them directly.

Composition also guides cleaning. Pearls (calcium carbonate) dissolve in acids; ultrasonic cleaners can drive oil out of emeralds and stress opals and fractured stones. Warm water, mild soap and a soft brush are safe for nearly everything.

Do lab-created gemstones have the same chemical composition?

Yes. A lab-created (synthetic) gem has the same chemistry, structure and properties as its natural counterpart; only its origin differs. Lab-created ruby and sapphire are Al₂O₃, made by flame fusion since the early 1900s, or by flux and other growth methods. Laboratory-grown diamonds are pure carbon, made by HPHT (high pressure, high temperature) or CVD (chemical vapour deposition). Neither is fake, and natural stones are not an investment product; they are simply rarer.

A simulant is different: it only looks like the gem and has a different chemical composition.

Natural and lab-created stones should always be disclosed as such on the invoice and report. For natural material, start with the stones currently available.

How do gemologists identify a gemstone's composition: step by step

You cannot see a chemical formula, so a gemologist reads it from the stone's behaviour. A typical bench identification runs like this:

1. Look with a 10× loupe and microscope. Inclusions, growth lines and fractures often point to the species and to natural or lab-created origin.
2. Take a refractometer reading. The RI and the gap between two readings (birefringence) narrow the options quickly; 1.762–1.770 says corundum, 1.577–1.583 says beryl.
3. Check optic character with a polariscope. This shows whether a stone is singly refractive (diamond, spinel, garnet, and amorphous materials such as opal and glass) or doubly refractive (sapphire, emerald).
4. Measure specific gravity for loose stones. Weighing in air and water separates look-alikes such as sapphire (about 4.00) from spinel (about 3.60).
5. Use a spectroscope and UV lamp. Absorption lines and fluorescence reveal colouring elements, for example chromium's strong red glow in many rubies.
6. Send important stones to a laboratory. Advanced testing confirms natural or lab-grown origin, treatments and, for some gems, geographic origin.