Beryllonite
As its name suggests, this mineral also contains beryllium, being a soda phosphate corresponding to the formula NaBePO4. Clear, colourless stones, which occur at Stoneham, Maine, U.S.A., have been cut, but the lack of ‘fire,’ the easy cleavage, and comparative softness, the symbol being 5½ on Mohs’s scale, unfit it for use in jewellery. The double refraction is biaxial in character and negative in sign, the least and the greatest of the refractive indices being 1·553 and 1·565 respectively.