The group
One species, two of the four traditional precious stones. The split is purely by colour and purely by convention: red corundum is ruby, everything else is sapphire, including pink, orange, yellow, green, purple, colourless and black. Where pink stops and red begins is a trade judgement and laboratories disagree at the margin.
Chromium makes the red. Iron and titanium together make the blue. The rare orange-pink balance of chromium and colour centres makes padparadscha, a name with no agreed boundary that adds a large premium to a colour nobody can define precisely.
Corundum carries the widest range of treatments of any gem family, and they are not equivalent. Heat is universal and accepted. Lattice diffusion with beryllium or titanium adds colour from outside and is a different product at a different price. Lead-glass filling of heavily fractured ruby produces a composite, not a treated stone, and it is fragile in ways natural ruby is not.
At 9 on the Mohs scale with no cleavage, corundum is the most practical of all coloured stones for daily wear — which is why sapphire, not tanzanite, is the honest answer when someone wants a blue stone for an engagement ring.
Every corundum compared
The whole group side by side. Where the constants overlap, the species cannot be named from a refractometer alone — which is exactly why the trade names mislead.
| Variety | Species | Colour | Mohs | RI | SG | Guide price |
|---|---|---|---|---|---|---|
| Anyolite | A metamorphic rock: green zoisite with pink-red corundum crystals and black pargasite amphibole | green | 6-7 zoisite, 9 corundum - an awkward hardness contrast for polishing | 1.69-1.70 (zoisite), 1.76-1.77 (corundum) | 3.00-3.40 | $3-20 per kilogram rough; carved figures and spheres $30-500 |
| Colour-change Sapphire | Corundum | blue | 9 | 1.762–1.770 | 3.95–4.05 | 200–3,000 USD/ct depending on the strength of the change |
| Corundum | Corundum | red | 9 | 1.762–1.770 | 3.95–4.05 | Abrasive corundum a few USD/kg; gem varieties from 10 to well over 100,000 USD/ct |
| Emery | A naturally occurring abrasive rock - an intergrowth of corundum with magnetite, hercynite spinel and hematite | black | 7-9 (the corundum controls the cutting) | 1.76-1.77 (corundum) | 3.70-4.30 | $200-700 per tonne for graded natural emery; the market is now very small |
| Padparadscha | Corundum | orange | 9 | 1.762–1.770 | 3.95–4.03 | $1,000–30,000/ct |
| Pink Sapphire | Corundum | pink | 9 | 1.762–1.770 | 3.97–4.05 | Commercial $100–$800/ct; fine unheated 2 ct+ $3,000–$15,000/ct |
| Ruby | Corundum | red | 9 | 1.762–1.770 | 3.97–4.05 | $100–100,000/ct |
| Ruby in Fuchsite | Corundum in chromian muscovite rock | red | 2.5–3 matrix; 9 corundum | Opaque aggregate | 2.8–3.1 | Rough 5–30 USD/kg; carvings 30–300 USD |
| Sapphire | Corundum | blue | 9 | 1.762–1.770 | 3.95–4.03 | $50–50,000/ct |
| Star Ruby | Corundum | red | 9 | 1.762–1.770 (spot ~1.76) | 3.97–4.05 | Opaque commercial $20–$150/ct; fine translucent Burmese $2,000–$20,000/ct |
| Star sapphire | Corundum | blue | 9 | 1.762–1.770 | 3.95–4.03 | $50–5,000/ct |
| Trapiche Ruby | Corundum | red | 9 | 1.762–1.770 | 3.95–4.05 | 100–2,000 USD/ct; sharp symmetrical patterns at the top |
| Verneuil Synthetic Corundum | Synthetic corundum | red | 9 | 1.762–1.770 | 3.98–4.00 | Under 1 USD/ct as rough boules; a few USD/ct cut |
| Yellow Sapphire | Corundum | yellow | 9 | 1.762–1.770 | 3.97–4.05 | Commercial heated $100–$600/ct; fine unheated Ceylon $1,500–$6,000/ct |
Questions
What is the difference between ruby and sapphire?
Nothing but colour. Both are the mineral corundum. Red corundum is called ruby; every other colour, including pink, is called sapphire. Where pink becomes red is a trade judgement, not a chemical boundary.
Is sapphire heated?
Usually. Heating is standard, accepted and priced in. What matters more is whether the stone has been lattice-diffused with beryllium or titanium, which adds colour from the surface inward and is worth far less — that requires a laboratory to detect.
What is a lead-glass filled ruby?
Heavily fractured low-grade ruby with lead glass forced into the fractures. It is a composite rather than a treated stone, it can be damaged by ordinary jewellery repair and household acids, and it is worth a small fraction of natural ruby. It must be disclosed.
Is sapphire good for an engagement ring?
It is the best coloured-stone answer there is. Hardness 9, no cleavage, no special care. It outlasts every other coloured gem in daily wear.
Looking for corundum?
Tell us which variety, what size and what it is for. We source it, photograph it and quote it, with the locality named.
Ask about corundum