Coral
Coral belongs to the same small category as pearl and amber: a gem material built by a living organism rather than formed through ordinary mineral crystallization. Precious red and pink coral, the type most associated with jewelry, is the skeleton secreted by colonies of soft-bodied marine polyps in the genus Corallium — over generations, those polyps build a branching, tree-like calcium-carbonate structure, and it’s that structure, harvested and polished, that becomes the gem material.
That structure is genuinely calcitic — the trigonal form of calcium carbonate, the same mineral behind limestone and marble — but it isn’t pure mineral in the way quartz or corundum is. An organic matrix runs through the calcite, similar in principle to how pearl’s aragonite platelets are held together by organic conchiolin, which is part of why coral is classified as an organic gem material rather than a true mineral despite having a mineral-based composition. At Mohs 3 to 4, it’s also considerably softer than most mineral gemstones — closer to a fingernail-scratchable material than to anything in the quartz or corundum families — which is why coral jewelry needs more careful handling than harder stones.
Not all coral used in jewelry shares that same calcitic chemistry. Black coral is a different material altogether: a horn-like, proteinaceous substance rather than a carbonate, built by an unrelated group of colonial organisms (Antipatharians). Blue coral is different again, built from aragonite rather than calcite. The figures recorded on this page describe the calcitic, red-and-pink material that dominates the gem trade, not these other types.
Red and pink coral has historically been sourced from the Mediterranean, particularly Mediterranean waters near Italy, with Japan and Taiwan also long-established centers of the coral jewelry trade. Coral harvesting is a genuinely sensitive area of the gem trade today: several coral species, including varieties of Corallium, are now protected or regulated under international conservation agreements, and legality of sourcing varies significantly by species and origin — worth knowing before buying, whatever the species.
Coral doesn’t appear on any of the historical or modern monthly birthstone lists, or on either zodiac system, tracked on this site — it’s included here as a standalone gemstone reference entry, the same way pearl is.
Gemmological properties
| Mohs hardness | 3–4 |
|---|---|
| Crystal system | Trigonal (calcite, in precious/red coral) |
| Chemical formula | CaCO3, with organic matrix |
| Refractive index | 1.486–1.658 |
| Color family | Red, Pink, White, Black |
| Variety of | — |
| Also known as | — |
| Origins | Italy, Japan, Taiwan |
| Organic material | Yes |
Precious red and pink coral (Corallium) is calcitic, but the trade also covers materials with different chemistry entirely — black coral is a horn-like protein, and blue coral is aragonite — so figures above describe the calcitic, gem-trade-dominant type specifically. Several coral species are now protected or CITES-regulated; legality of sourcing varies by species and origin.
Frequently asked questions
- Is coral a mineral?
- No. Like pearl and amber, it's an organic gem material — built by living marine organisms rather than formed through geological crystallization. Precious red and pink coral is the calcium-carbonate skeleton secreted by colonies of Corallium polyps.
- Why does coral's data mix a mineral crystal system with an organic material?
- Because precious coral's skeleton is genuinely calcitic — the same mineral, calcium carbonate in its trigonal form, that makes up limestone and marble — but it's built by a living organism into a branching structure with an organic matrix woven through it, the same way pearl's aragonite is organized around organic conchiolin. The figures on this page describe that calcitic, gem-trade-dominant type specifically; black coral is a different material entirely, a horn-like protein rather than a carbonate.
- Is coral a birthstone on any of the lists this site tracks?
- No. It doesn't appear on any of the historical, US, UK, or zodiac systems documented on this site — it's covered here as a standalone gemstone reference entry.