Zircon

Zircon is the oldest mineral on Earth. The Jack Hills zircons from Western Australia have been dated at 4.4 billion years old - formed just 150 million years after Earth itself. These tiny crystals survived the Hadean eon, the period of intense meteorite bombardment and magma oceans, and preserve evidence of conditions on the earliest Earth, including signatures suggesting liquid water existed far earlier than previously thought.

Zircon crystallizes from magma as an accessory mineral in igneous rocks, particularly granites and pegmatites. It's extremely resistant to weathering and alteration, which is why it survives billions of years of geological recycling. Zircon is also the primary mineral used for uranium-lead radiometric dating - the technique that determined Earth's age.

Gem-quality zircon comes primarily from Cambodia, Sri Lanka, and Myanmar. Natural colors include yellow, orange, red, brown, and green. Blue zircon - the most popular gem variety - is produced by heating brown or reddish zircon from Cambodia.

Zircon is identified by its high luster (adamantine - diamond-like), strong double refraction (visible doubling of back facet edges when viewed through the stone), and high specific gravity (4.70 - it feels noticeably heavy). The tetragonal crystal habit produces square prism shapes.

Distinguish from cubic zirconia (synthetic, singly refractive, different density), diamond (singly refractive, harder), and aquamarine (lower SG, lower luster). Zircon's doubled back facets visible through the crown are the quickest gemological test.