Yellow Sapphire

Yellow sapphire is the yellow variety of corundum, aluminium oxide, coloured by traces of iron; at hardness 9 it shares the extreme durability of all sapphire. Much commercial yellow sapphire is heat-treated to deepen or clarify the colour, and some pale material is irradiated, so disclosure matters in the trade. Its single most important traditional role is in jyotisha, where under the name pukhraj it is the gem of Jupiter, Guru or Brihaspati, and one of the nine navaratna stones; it is among the most commonly prescribed astrological gems in India, worn to strengthen Jupiter's benefic influence on wisdom, prosperity and marriage. That is a living, documented tradition with a substantial prescriptive literature. Yellow sapphire is also one of the September birthstones as a colour variant of sapphire. Its broader metaphysical associations largely restate the jyotisha character of Jupiter in modern terms.

Yellow sapphire is corundum, aluminium oxide, the same mineral as ruby and as blue sapphire, and almost everything about its appearance on the market turns on a treatment question.

Corundum is colourless when pure. Trace elements do the rest: chromium gives red and the stone is called ruby, iron and titanium together give blue, iron alone tends toward yellow, and beryllium introduced deliberately can shift the colour further. The mineral is the second hardest in common use after diamond, which is why it survives in river gravels and why the historic sources are alluvial deposits in Sri Lanka, Burma, Thailand and Madagascar.

Heat treatment of corundum is ancient and near-universal. Heating alters the state of the colour-causing elements and dissolves the fine rutile inclusions that make a stone cloudy, and the great majority of sapphire on the market has been heated. This is disclosed as a category rather than stone by stone, and it is accepted in the trade. What is contested is lattice diffusion, in which an element is driven into the surface of the stone at high temperature to create or deepen colour, since the resulting colour may be only skin-deep and a recut can remove it.

That distinction matters here beyond the commercial. Jyotish practice, which places yellow sapphire with Jupiter among the nine stones of the navaratna, holds views about whether a treated stone is acceptable, and those views are held by practitioners and vary between them. The atlas records the practice as existing rather than adjudicating it.