Intermediate zircon, sometimes written as medium zircon, denotes a category that sits between the two extremes of the zircon family, fully crystalline high zircon at one end and almost entirely metamict low zircon at the other. The classification is based on the degree to which radioactive decay of trace uranium and thorium has disrupted the crystal lattice over geological time, and it is reflected in measurable changes to refractive index, density and birefringence.

The metamict process

Zircon almost always contains traces of uranium and thorium that substitute into the zirconium site. As these elements decay, alpha particles and recoil nuclei progressively damage the surrounding lattice. Over hundreds of millions of years the damage accumulates and the structure begins to break down, becoming amorphous in patches. A specimen that has experienced enough decay to record substantial damage but has not yet lost its crystallinity altogether is called intermediate.

Diagnostic properties

High zircon, the freshly crystalline variety, is the standard gemmological reference, with a refractive index near 1.92 to 2.01, birefringence near 0.059 and a specific gravity around 4.7. Low zircon, the most damaged form, drops to refractive index of about 1.78 to 1.82, birefringence near zero and SG around 4.0. Intermediate specimens fall between these limits in all three properties simultaneously. The relationship between RI, SG and the degree of metamictisation is monotonic, so a careful refractometer and hydrostatic-balance reading can place a stone in the high, intermediate or low range.

Visual and absorption clues

Brown and greenish-brown zircon is statistically more often intermediate or low than is sky-blue heated zircon, which is typically high. The familiar uranium absorption lines, particularly the strong line at 653.5 nanometres, are sharp and narrow in high zircon and become progressively diffuse as the structure becomes more disordered. A spectroscope reading that shows broad rather than sharp absorption is a useful pointer to an intermediate or low specimen.

Heating and conversion

Intermediate zircon can be partially restored by heating, which encourages recrystallisation of the damaged lattice. The blue zircon ubiquitous in the trade is produced by heating brownish material from Cambodia, Tanzania and elsewhere; the heat treatment both develops the colour and tends to push the structure back towards the high end of the range. Stones that emerge stable and high after heating are indistinguishable in their physical properties from naturally crystalline material.

Trade implications

The intermediate label is rarely used at retail, but it matters for identification and for predicting durability. More heavily metamict stones are softer, easier to chip and prone to cleavage along damage planes. A stone with low birefringence relative to its refractive index, or with diffuse absorption lines, should be handled with the gentleness appropriate to a partially amorphous structure rather than a fully crystalline one.