The babe among gem-stones, benitoite first saw the light of day a few years ago, early in 1907. It occurs with the rare mineral neptunite, which was previously known only from Greenland, in narrow veins of natrolite in Diablo Range near the head-waters of the San Benito River, San Benito County, California. Despite careful search the species has not been found except within the original restricted area. To science it is interesting both because of its composition, a silico-titanate of barium, corresponding to the formula BaTiSi3O9, and because its crystals belong to a class of crystalline symmetry which has hitherto not been represented among minerals. The double refraction is uniaxial, and since the ordinary index of refraction is 1·757 and the extraordinary 1·804, it is positive in sign and large in amount, namely, 0·047. The stones are characterized by strong dichroism, the colour corresponding to the ordinary ray being white, and to the extraordinary greenish blue to indigo depending upon the tint of the stone. To obtain the best effect the stone must therefore be cut with the table-facet parallel to the crystallographic axis. The specific gravity is 3·65, and hardness 6½ on Mohs’s scale. When first discovered the species was supposed to be sapphire, and many stones were cut and sold as such. It is, however, much softer than sapphire, and is readily distinguished by its optical characters, since it possesses greater double refraction and of differing sign, so that, when tested with the refractometer, the shadow-edge corresponding to the lower index of refraction remains fixed in the case of benitoite, whereas the contrary happens with sapphire. Benitoite also, unlike sapphire, fuses easily to a transparent glass. Its blue colour, which is supposed to be due to a small amount of free titanic acid present, appears to be stable. Several stones as large as 1½ to 2 carats in weight have been found. The largest of all, perfectly flawless, weighs just over 7 carats, and is remarkable because it is about three times the next largest in point of weight; it is the property of Mr. G. Eacret, of San Francisco.
CHAPTER XXXV
EUCLASE, PHENAKITE, BERYLLONITE
Euclase
THIS species comes near beryl in chemical composition, being a silicate of aluminium and beryllium corresponding to the formula Be(AlOH)SiO4, and closely resembles aquamarine in colour and appearance when cut. Owing to the rarity of the mineral good specimens command high prices for museum collections, and it is seldom worth while cutting it for jewellery. It derives its name from its easy cleavage, εὖ easily, and κλάσις fracture. The double refraction is biaxial in character and positive in sign, the least and greatest of the refractive indices being 1·651 and 1·670 respectively; the specific gravity is 3·07, and the hardness 7½ on Mohs’s scale. The colour is usually a sea-green, but sometimes blue. Euclase occurs with topaz at the rich mineral district of Minas Novas, Minas Geraes, Brazil, and has also been found in the Ural district, Russia.
Phenakite
Another beryllium mineral, phenakite owes its name to the frequency with which it has been mistaken for quartz, being derived from φέναξ, deceiver. The clear, colourless crystals, somewhat complex in form, have at times been cut, but they lack ‘fire,’ and despite their brilliant lustre meet with little demand. The composition is a silicate of beryllium corresponding to the formula Be2SiO4. The double refraction is uniaxial, and since the ordinary, 1·652, is less than the extraordinary index, 1·667, it is positive in sign; the specific gravity is 2·99, and the hardness is almost equal to that of topaz, being about 7½ to 8 on Mohs’s scale.
Fine stones have long been known near Ekaterinburg in the Ural Mountains, and have recently been discovered in Brazil.