A good blue sapphire of 10 carats is valued at $260. it it weighs 20 carats, its value is $1040 ; but under 10 carats, the price may be estimated by multiply ing the square of its weight in carats into a quarter eagle ; thus, one of four carats would be worth X E. $40. It has been said that the blue sapphire is su perior iu hardness to the red, but this is probably a mistake arising from con founding the corundum ruby with the spindle ruby. A sapphire of a barbel blue color, weighing 6 carats, was dis posed of in Paris by public sale for $350 ; and another of an indigo blue, weighing 6 carats and 3 grains, brought $300: both of which sums much exceed what the preceding rule assigns, from which we may perceive how far fancy may go in such matters. The sapphire of Brazil is merely a blue tourmaline, as its spe cific gravity and inferior hardness show. White sapphires are sometimes so pure that when properly cut and polished they have been passed for diamonds.
The yellow and green sapphires are much prized under the names of Orien tal topaz and emerald. The specimens which exhibit all these colors associated in one stone are highly valued, as they prove the mineralogical identity of these varieties.
Besides these shades of color, sap phires often emit a beautiful play of co lors, or chntoiement, when held in differ ent positions relative to the eye or inci dent light ; and some likewise present star-like radiations, whence they arc called star-stones or eateries ; sending forth 6 or even 12 rays, that change their place with the position of the stone. This property, so remarkable in certain blue sapphires, is not, however, peculiar to these gems. It seems to belong to transparent minerals which have a rhom boid for their nucleus, and arises from the combination of certain circumstances in their cutting and structure. Lapida ries often expose the light-blue variety of sapphire to the action of fire, in order to render it white and more brilliant ; but with regard to those found at Ex pailly, in France, fire deepens their color.
3. Cflirroberyl, called by Haiiy, Cymo phane, and by others, Prismatic corun dum, ranks next in hardness to sapphire, being 8.5 on the same scale of estimation. Its specific gravity is 3.754. It usually occurs in rounded pieces about the size of a pea, but it is also found crystallized in many forms, of which 8-sided prisms with 8-sided summlis are perhaps the most frequent. Lustre vitreous, color asparagus green, passing into greenish white and olive-green. It shows a bluish opalescence, alight undulating, as it were, in the stone when viewed in certain di rections ; which property constitues its chief attraction to the jeweller. When
polished, it has been sometimes mistaken tor a yellow diamond; and from its hard ness and lustre is considerably valued. Good specimens of it are very rare. It has been found only in the alluvial de posites of rivers, along with other species of gems. Thus it occurs in Brazil, along with diamonds and prismatic topaz ; also in Ceylon. Its constituents are alumina, 68.66; glucina, 16.00; silica, 6.00; pro toxide of iron, 4.7 ; oxyde of titanium, 2.66; moisture, 0.66 ; according to Sey bert's analysis of a specimen from Brazil. It is difficultly but perfectly fusible before the blow-pipe, with borax and salt of phosphorus. In composition it differs entirely from sapphire, or the rhombohe dral corundum.
4. Spinelle Ruby, called Dodecahedral corundum, by some mineralogists, and Balas ruby, by lapidaries: Its hardness is 8. Specific gravity, 3.523. Its funda mental form is the hexahedron, but it oc curs crystallized in many secondary forms : octahedrons, tetrahedrons, and rhombo hedrons. Fracture, conchoidal ; lustre, vitreous ; color, red, passing into blue and green, yellow, brown, and black ; and sometimes it is nearly white. Red spi nelle consists of alumina, 74.5; silica, 15.5; magnesia, 8.25 ; oxide of iron, 1.5; lime, 0.75. Vauquelin discovered cent. of chromic acid in the red spinelle. The red varieties exposed to heat become black and opaque ; on cooling, they ap pear first green, then almost colorless, but at last resume their red color. Pleonaste is a variety which yields a deep green globule with borax.
Crystals of spindle from Ceylon have been observed imbedded in limestone, mixed with mica, or in rocks containing adularia, which seem to have belonged to a primitive district. Other varities like the pleonaste occur in the dntsy cavities of rocks ejected by Vesuvius. Crystals of it are often found in diluvial and alluvial sand and gravel, along with true sap phires, pyramidal zircon, and other gems; as also with octahedral iron ore, in Cey lon. Blue and pearl-gray varieties occur in Siidermannland, in Sweden, imbedded in granular limestone. Pleonaste is met with also in the diluvial sands of Ceylon. Clear and finely colored specimens of spi nelle are highly prized as ornamental stones. When the weight of a good spi nelle exceeds 4 carats, it is said to be val ued at half the price of a diamond of the same weight. M. Brard has seen one at Paris which weighed 215 grains.