silicates
Zircon
Zr(SiO4)
- Also known as
- hyacinthjargonstarlite
- Crystal system
- tetragonal
- Hardness
- 6–7.5 on the Mohs scale
- Lustre
- adamantine
- Colour
- Brown, red, yellow, blue, colourless
- Where it forms
- In granites and metamorphic rocks, as a minor accessory. It is exceptionally durable and survives erosion, transport and even remelting.
- Chiefly from
- CambodiaSri LankaAustraliaTanzania
Why it is that colour
Impurities and radiation damage. Zircon incorporates uranium and thorium when it forms, and their decay progressively wrecks the crystal structure — which changes the colour, lowers the hardness and is called metamictisation.
Safety
Zircon contains uranium and thorium and is very slightly radioactive. The dose from a gemstone is negligible and not a practical concern; heavily metamict material is more active and should not be kept in bulk against the body. Blue zircon is heat-treated, which is standard and disclosed.
Where it comes from
Mined from heavy mineral sands for zirconium metal and ceramics, with gem material from Cambodia, Sri Lanka and Tanzania as a small specialist trade.
Historical use
The oldest known piece of the earth’s crust is a zircon crystal from the Jack Hills of Western Australia, dated at about 4.4 billion years. The rock it was in is much younger; the zircon survived being eroded out of its parent and reburied.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Known as hyacinth or jacinth in medieval lapidaries and biblical translations, where it is credited with promoting sleep and warding off plague. Modern meanings are recent, and it has been substantially displaced in public awareness by cubic zirconia, an unrelated synthetic.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Sold as this, but not
Worth knowing
Zircon is how we know how old the earth is, and it works because of a happy chemical accident. When zircon crystallises it readily accepts uranium into its structure and strongly rejects lead. So any lead found inside a zircon crystal must have arrived by radioactive decay of the uranium, and the ratio gives the age directly.
They are also nearly indestructible. Jack Hills zircons in Western Australia have been dated to about 4.4 billion years — within roughly 150 million years of the planet forming — and their oxygen isotopes suggest they crystallised in the presence of liquid water, which pushed back the estimated date of the first oceans considerably.
Note the confusion the trade has bequeathed here. Zircon is a natural mineral with a long history. Cubic zirconia is a laboratory-made diamond simulant introduced commercially in the 1970s. The names are similar and the materials have nothing to do with each other.
Identity checked against IMA-CNMNC master list, June 2025.