Ruby

Ruby and sapphire are the same mineral. Both are corundum, aluminium oxide, and the only difference is colour: corundum coloured red by chromium is called ruby, and corundum of any other colour is called sapphire, qualified by that colour. There is no mineralogical boundary, and where exactly pinkish-red stops being ruby and starts being pink sapphire is a trade convention that differs between markets, with American practice historically stricter than Asian.

Corundum is 9 on the Mohs scale, second only to diamond among natural materials in common use, and it is tough as well as hard, with no cleavage. This makes it an excellent stone for wear and also an industrial abrasive; emery is impure corundum.

The chromium that reddens ruby also makes it fluoresce under ultraviolet, so fine ruby glows in sunlight and appears to burn from within. The same property put ruby at the centre of twentieth-century physics: the first working laser, built by Theodore Maiman in 1960, used a synthetic ruby rod.

Synthesis matters commercially. Auguste Verneuil's flame-fusion process, published in 1902, produced synthetic ruby in quantity and cheaply. Synthetic and natural ruby are chemically and structurally identical, distinguishable only by growth features. A very large proportion of red stones in older costume and even fine jewellery is Verneuil material, and much natural ruby on the market is heat-treated or has had fractures filled with lead glass, a treatment that is disclosed inconsistently.

The classic source is Mogok in Burma; Mozambique is now a major producer.

In Sanskrit ruby is ratnaraj, king of gems, and in jyotisha it is manikya, the stone of Surya, the Sun, and the first of the navaratna. That is a substantial and consistent tradition, and it is the source of most of the ruby lore that reaches Western books at second hand.

The Western material is messier, for the reason that runs through all of this: the names are unreliable. Before mineralogical analysis, red stones were sorted by appearance, and ruby, red spinel and garnet were not reliably distinguished. The consequences are concrete and sometimes spectacular. The Black Prince's Ruby in the British Imperial State Crown is a spinel. The Timur Ruby is a spinel. Balas ruby, a term that appears throughout medieval and early modern inventories and refers to material from Badakhshan, means spinel. Spinel was not recognised as a separate species until the late eighteenth century, and any red stone named in a source earlier than that should be treated as unidentified until examined.

This affects the correspondence literature directly. Where a medieval lapidary assigns virtues to carbunculus or to ruby, there is no way to know which mineral was in front of the writer, and the virtues attributed (protection in battle, staunching of blood, warning of danger by darkening), attach to redness rather than to a species.

Agrippa's Behenian list pairs Aldebaran with ruby or garnet, treating the two as alternatives, which is a Renaissance author acknowledging exactly this uncertainty rather than resolving it.

Ruby is the July birthstone on the 1912 trade list.