290 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



these localities the pure mineral often forms the nucleus of large masses of common 

 corundum. In the Culsagee mine on Corundum Hill, near Franklin in Macon County, a 

 crystal weighing 311 pounds was once found. This, however, was not of gem quality and 

 was coloured partly red and partly blue. At the same mine rubies, sapphire, " oriental 

 topaz," and a few " oriental emeralds," &c., suitable for cutting as gems, have been found. 

 Fine star-stones occur here also, as well as in Delaware County, Pennsylvania. 



The other region which is specially rich in precious corundum is sil uated in the west. 

 Sapphire and other colour-varieties of corundum have been known since 1865 to occur in 

 the neighbourhood of Helena on the upper reaches of the Missouri river, in the State of 

 Montana, being first discovered during the process of gold-washing. Again the true nature 

 of the stones was not at first recognised, and they were sold at much below their actual 

 value. Since 1891 these deposits have been systematically worked for gold, and at the 

 same time large numbers of the precious stones have been collected. They are found in 

 masses of glacial debris known as " bars," which are laid down on the sides of the valleys, 

 parallel to the river-courses and at a height of 300 feet above the presenh high-water level 

 of the upper Missouri. These glacial sands and gravels containing gold overlie black 

 shales, probably of Lower Silurian age, which are associated with limestones, quartzite, and 

 rocks of igneous origin. It is in the lowest layer of these sands and gravels, with a thickness, 

 of only a few inches, that the sapphire is principally found. The sapphires are most 

 abundant at Eldorado Bar, Spokane Bar, French Bar, and Ruby Bar, and these deposits- 

 are still being worked. Spokane Bar near Stubb's Ferry, twelve miles to the east of Helena, 

 is approximately the central point of this district, which extends along the Missouri for 

 at least fifteen miles and embraces an area of certainly no less than eleven and a half square 

 miles. 



The sapphires frequently occur as well-developed crystals, having the form of a short 

 hexagonal prism with basal planes, an unusual type for this gem. Irregular grains are also 

 found which, like the crystals, are more or less rounded. Neither crystals nor grains attain 

 to any considerable size, measuring at the most from ^ to J inch in diameter and rarely 

 exceeding 9 carats in weight. Though small in size the stones are abundant in number, as 

 evidenced by the fact that an acre of the deposit at Eldorado Bar yielded no less than 2000' 

 ounces of sapphire. Many of these stones, however, would be unsuitable for cutting, since- 

 the predominant tints of the sapphires of this locality are all pale. 



The colours, though almost always pale in shade, show great variety of tint, red,, 

 violet, yellow, blue, green, bluish-green, and all possible intermediate colours being met 

 with. Bluish-green and green corundum is specially abundant, while the pure blue and the 

 red varieties are absent. Occasionally a stone with a red nucleus and a border of another 

 colour is met with. Some green and blue stones appear red by artificial light. Almost all 

 the colour- varieties of corundum from this region, which are suitable for cutting, have a. 

 peculiar metallic sheen, which is very characteristic and is not seen in stones from any other 

 locality. They are remarkable also for the brilliancy of their lustre, and, according to the 

 statements of lapidaries, are specially hard. 



Corundum is associated in these glacial sands with many other minerals, among which 

 are crystals of white topaz not exceeding I inch in length, fine ruby-red garnets the size or 

 a pea (which have often been mistaken for true rubies), kyanite, cassiterite in small, rounded 

 grains (stream-tin), iron-pyrites altered to limonite, chalcedony, and small rounded fragments 

 of calcite. 



As already mentioned, the rocks occurring in situ in the district and underlying the 

 gemmiferous sands are penetrated by dykes of igneous material. In one of these dykes. 



