The carbonates

What holds the family together

Carbonates are minerals built around the CO3 group, and that group is their weakness: introduce acid and it releases carbon dioxide, which is the fizz you see when vinegar meets limestone. Every stone on this page will do that.

Malachite and azurite are copper carbonates, and they are chemically close enough that azurite slowly converts to malachite in the presence of water. That conversion is why some medieval paintings have skies that have turned green, and why a specimen of azurite kept in humid conditions may dull over decades.

Rhodochrosite is manganese carbonate, banded rose and white where it grew in layers in a cavity. Calcite is calcium carbonate, the most polymorphic common mineral, appearing in hundreds of crystal habits, and its clear variety Iceland spar shows double refraction strongly enough to split a printed line in two.

Rhodonite is the family's guest. It is a manganese silicate rather than a carbonate, harder at 6, and it is grouped here because it is rhodochrosite's constant companion in the market and in the mine.

How it forms

Copper carbonates form in the oxidised zone above a copper deposit, where groundwater carrying dissolved copper reacts with carbonate rock. That is why malachite and azurite occur together and why they are found capping copper mines from Katanga to Arizona to the Ural mountains.

The Congolese deposits supply most of the world's banded malachite today. Historically, Egypt worked malachite in Sinai and the Eastern Desert from the Predynastic period, grinding it for the green eye paint that appears in every tomb painting, and Russia's Ural deposits supplied the enormous decorative pieces of the eighteenth and nineteenth centuries, including whole malachite rooms.

Rhodochrosite forms in hydrothermal veins with silver and manganese ores. The banded stalactitic material comes from Capillitas in Argentina, from Inca-period workings, and superb crystals come from the Sweet Home mine in Colorado.

Calcite forms almost everywhere carbonate rock exists, and Iceland spar was quarried at Helgustadir in Iceland.

The stones of this family

• Malachite (Cu₂CO₃(OH)₂, Mohs 3.5). Banded copper carbonate, ground for Egyptian eye paint and for green pigment across Europe. Toxic as dust and in water, and beautiful in every other respect.
• Azurite (Cu₃(CO₃)₂(OH)₂, Mohs 3.5). Deep blue copper carbonate that slowly turns green as it converts to malachite, which is why medieval skies have gone the wrong colour in many paintings.
• Rhodochrosite (MnCO₃, Mohs 3.5). Rose banded manganese carbonate, the national stone of Argentina, found in Inca-era mines. Soft, soluble in acid, and never to be worn in daily rings.
• Calcite (CaCO₃, Mohs 3). The most polymorphic common mineral, and the source of Iceland spar, whose double refraction is the strongest candidate for the Viking sunstone.
• Rhodonite (MnSiO₃, Mohs 6). Pink manganese silicate veined with black oxide. Grouped here with its manganese cousin rhodochrosite, though it is chemically a silicate and much harder.

Trade names and lookalikes

Malachite is stabilised with resin very often, since much natural material is friable, and this is normal. Reconstituted malachite, meaning powder in a binder, also exists and looks too regular under a loupe. Genuine banded malachite has irregular concentric bands of varying width.

Dyed howlite and dyed magnesite are sold as green and pink stones across this family's price range.

Rhodochrosite is frequently reconstituted, which shows as a uniform grainy texture with no true banding.

Calcite is dyed in every colour imaginable and sold under invented names; natural calcite colours are white, honey, orange, green, blue and pink, and the dyed material is usually far more saturated.

The most useful check for this whole family is a discreet acid test, which is destructive and best left to a specimen you own outright: a drop of dilute acid on an inconspicuous spot will fizz on a carbonate and not on a silicate.

Care that follows from the structure

This is the family where care instructions matter most.

Never put a carbonate in acid of any kind, which includes vinegar, lemon juice, most jewellery dips, cola and swimming pool water. Never use an ultrasonic cleaner. Clean with a dry or barely damp soft cloth.

The copper members carry a real toxicity note. Malachite, azurite and chrysocolla release copper compounds in water and as dust. A polished sealed piece handled normally is safe. Sanding, grinding or tumbling them without dust control is not, and no copper mineral should ever be used to make a drinking elixir by direct contact.

Rhodochrosite at 3.5 will scratch against almost anything and is a pendant stone rather than a ring stone. Calcite at 3 is a display mineral.

Rhodonite at 6 is the only member robust enough for daily wear.