Introduction to Ruby and Sapphire Gemstones
Ruby and the different colours of sapphire are colour varieties of the mineral corundum, aluminium oxide (Al2O3). Entirely colourless corundum is not common in Nature as iron (the best example) is rarely far from the action when minerals are forming so that faceted colourless corundum must be tested for clues to its origin. Small amounts of chromium are the cause of the red colour in ruby and iron plays a similar part in the colouration of green and yellow sapphire. Blue sapphires contain iron and titanium, both being necessary for colouration as we shall see below. Chromium does not always give red; orange-pink or pinkorange specimens from Sri Lanka are called padparadscha and command high prices. Purple and mauve sapphires are known and highly desirable; brown sapphires can also be attractive if not too dark. Star rubies and blue sapphires at their best can be magnificent and named specimens are known. Titanium dioxide (rutile) is essential for star formation and as it is not found in sufficient quantities (if at all) in other colours of corundum the absence of asteriated yellow specimens, for example, is explained.
Cat’s eyes are not common but can form if the inclusions are appropriately distributed. Some sapphires may show a colour change though any specimen known to be corundum and showing a change from mauve to purple will, virtually invariably, turn out to be a vanadium-doped flamefusion synthetic product. The root rub suggests red in a number of languages but careful study is necessary if the derivation of today’s names (for any natural product) needs to be traced back to Classical or other literatures. In his translation of Pliny’s Naturalis Historia, Eichholz (1962) [Loeb Classical Ruby and Sapphire tells us that sappirus is not the sapphire of today; further efforts towards the investigation of corundum (or any other) varietal nomenclature should be directed towards the philologist rather than the mineralogist! The same can be said of the considerable amount of information available on the lore and legend of corundum. It is likely that some of the names have developed from Sanskrit roots. A translation of the thirteenth-century Ahmad ibn Yusuf Al Tifashi’s Best Thoughts on the Best of Stones, incorporated in Samar Najm abul Huda’s Arab Roots of Gemology, 1998 is a good example of the type of study that may be needed for similar non-European MSS. Named rubies include the De Long star ruby, in the American Museum of Natural History, New York, weighing 100 ct and the Rosser Reeves star ruby, in the National Museum of Natural History, Smithsonian Institution, Washington DC, USA, 138.7 ct (stone from Sri Lanka).
Named blue sapphires include the 423 ct Sri Lankan Logan sapphire, in the Smithsonian Institution, and the Myanmar Star of Asia, 330 ct, in the same museum. The Black Star of Queensland (733 ct, in a private collection) is the largest known black star sapphire. The Star of India, in the American Museum of Natural History, is a blue 536 ct sapphire.Other examples are quoted by Arem in the second edition of Color Encyclopedia of Gemstones, 1987. Streeter’s own short connection with the mines.