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Silicate

Zircon

ZrSiO₄

Earth's oldest mineral — and a brilliant gem older than any rock it sits in.

Hardness7.5
Crystal systemTetragonal
LusterAdamantine
Sp. gravity4.6–4.7
ClassSilicate
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Zircon specimen

Zircon — Photo: Nkansah Rexford · CC BY 3.0

What is Zircon?

Zircon is zirconium silicate — ZrSiO₄ — and it owns two very different reputations. To gem dealers it is a high-dispersion stone once sold as a diamond substitute. To geologists it is the most important mineral on Earth: zircon survives everything, locks uranium-lead ages inside, and the oldest dated grains — 4.4 billion years — are the oldest objects ever found on this planet.

Trace uranium and thorium inside zircon slowly wreck its own lattice (metamictization); gem-grade 'high' zircon keeps its structure, while metamict 'low' zircon loses density and luster.

In one sentence: Dense tetragonal prisms with diamond-fire — and a built-in atomic clock.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Geochronology. Zircon excludes lead when it grows but accepts uranium — the accumulated Pb/U ratio records its age.

Metamictization. Radiation damage lowers SG and RI from 'high' toward amorphous 'low' zircon.

Dispersion. 0.039 — enough fire to masquerade as diamond in old jewelry.

What It’s Used For

GeochronologyU–Pb dating of zircon is geology's master clock — crystals date Earth itself.
GemstoneHeat-treated blue zircon is a December birthstone.
Zirconium sourceZircon sands supply zirconium for ceramics and reactors.

How to Identify It

Tetragonal prisms with pyramidal endsVery dense (SG ~4.65)Adamantine luster, high dispersionCommon as small grains in sands
Looks like…But differs because…Quick test
DiamondMuch harder, octahedral habit, no tetragonal formHardness & habit
TopazOrthorhombic, perfect basal cleavage, lighterCleavage & density

Where It’s Found

Notable localities for fine specimens:

Jack Hills, Australia Mud Tank, Australia Ratanakiri, Cambodia Ural Mountains, RussiaOpen the locality map →