Montana Yogo Sapphire: Gemstone Information

  Yogo blue sapphires are almost recognizable from their exceptionally bright but deep colour; as crystals are not usually large, Yogo sapphire make superb melee. Lilac and purple stones can also be beautiful; a very few rubies have been found. Without doubt Yogo sapphires are the most commerically important of all the coloured stones so far found in North America. Price guides for any gem species are notably unreliable owing to the market’s inevitable volatility but, in Standard Catalog of Gem Values, second edition, 1984 (ISBN 0945005164), Miller and Sinkankas suggest US$ 1000–3500 for fine blue Yogo sapphires weighing 0.5–2 ct. In The 1922 Handbook and Descriptive Catalog of the Collections of Gems and Precious Stones in the United States National Museum (1922) Merrill lists only two blue sapphires under the heading Yogo Gulch and in Corundum Deposits of Montana (United States Geological Survey Bulletin 983, 1952) Clabaugh mentions a price of $75 a carat for the larger Yogo sapphires when faceted. This is a vital report for anyone interested in Montana corundum in general. In Gemstones, Quality and Value, vol. 1 (1993; ISBN 4418999027) Yasukazu Suwa says that Montana blue sapphires are a watery blue and do not command the highest prices for that reason. He does not say, however, that the sapphires described are Yogo rather than from one of the alluvial deposits – Yogo sapphires at their best are far from watery in appearance. Crystals from Yogo are illustrated in extraLapis 15 (1998) the photographs showing the crystallographic nature of eight typical specimens. Sapphires showing a wide range of colours are found in the (dry, overlain) gravels of the Missouri River in the general neighbourhood of Helena, Montana. While the colours can be very attractive, most of the production is routinely heated to give brighter colours. In general, crystals are barrel-shaped though some are flattened and characteristically striated across the flat pinacoid faces. Colours usually sold are blue, green and yellow. A fine ‘hot pink’ is achieved by heating and, in general, the treated stones are most attractive. Details of the results obtained by heating can be found in papers by John L. Emmett and Troy R. Douthit, who have made a close study of heating methods and their results. Yogo sapphire are not heated. The main working sites are Rock Creek, Dry Cottonwood Creek and Eldorado Bar. It is interesting and instructive to study the efforts made by a number of individuals and companies over the years to establish a serious and profitable trade in the heated sapphires. There has been limited and local success but the sapphires are not known in Europe or even very well in the US; though the position could improve if only the supply of sapphires from South and south-east Asia and Australia was not so well established. Sapphires, apart from Yogo specimens, known to be from Montana are so routinely heated that disclosure is probably not needed though if there were to be a regular export of sapphires from North America some form of certification might be called for. When not heated the sapphires from the gem gravels may show negative crystals occupied in some instances by natural glass and one or more gas bubbles. Heating alters inclusions and obscures their outlines. In North Carolina ruby of fair quality has been found in Macon County, at Cowee Creek. Ruby crystals are found in gravels with red garnet. Rubies with rutile inclusions have been cut to give star stones of good quality