White Sapphire

White or colourless sapphire is pure corundum, aluminium oxide, without the trace elements that make its coloured varieties: it is the same species as ruby and blue sapphire, differing only in the absence of chromium, iron or titanium. At hardness 9 it is second only to diamond, exceptionally durable, and has long served as a diamond substitute, more convincingly than glass or cubic zirconia because it is a genuine hard natural gem, though it lacks diamond's fire owing to lower dispersion. Much white sapphire on the market is heat-treated to remove a faint tint.

In Indian jyotisha colourless sapphire is used as a substitute for diamond, the gem of Venus, Shukra, when diamond is beyond reach, a recognised practice within the navaratna tradition. Beyond that its symbolism is thin and largely borrowed; modern metaphysical writing assigns it general themes of clarity and the higher centres on the basis of its colourlessness.

White sapphire is corundum with none of the trace elements that colour it, and its commercial history is almost entirely a story about synthesis.

Colourless corundum was long used as a diamond substitute. It is hard, it takes a good polish, and it is durable enough to survive daily wear. What it lacks is dispersion: diamond splits white light strongly into spectral colours, and corundum does so only weakly, so a white sapphire looks glassy where a diamond looks lively. That difference is visible to an unaided eye once it is pointed out.

The decisive event was the Verneuil process, published in 1902, in which powdered alumina is dropped through an oxyhydrogen flame onto a rotating pedestal, where it fuses into a single boule of synthetic corundum. It was the first commercially successful method of making gem-quality synthetic crystal, and it made corundum of any colour available cheaply and in quantity. Synthetic ruby and sapphire flooded the market within a decade, and gemmology as a professional discipline grew directly out of the need to tell them apart.

Synthetic corundum also went into things that are not jewellery. Watch bearings, the jewels of a jewelled movement, are synthetic corundum. So are laser rods, scratch-resistant windows and hard-wearing instrument surfaces.

The traditional attributions recorded elsewhere in this atlas were all formed when a colourless corundum was a rare natural stone. Nothing in them anticipates a furnace that can produce one on demand, and the atlas notes the gap rather than papering over it.