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.