The symmetry is orthorhombic, and the crystals are prismatic in shape and terminated by numerous inclined faces, and usually by a large face perpendicular to the prism edge (Fig. 72). Topaz cleaves with great readiness at right angles to the prism edge; owing to its facile cleavage, flaws are easily started, and caution must be exercised not to damage a stone by knocking it against hard and unyielding substances. The dichroism of a yellow topaz is always perceptible, one of the twin colours being distinctly more reddish than the other, and the phenomenon is very marked in the case of stones the colour of which has been artificially altered to pink. The values of the least and the greatest of the principal indices of refraction vary from 1·615 to 1·629, and from 1·625 to 1·637, respectively, the double refraction being about 0·010 in amount, and positive in sign. The high values correspond to the altered stones. The specific gravity, the mean value of which is 3·55 with a variation of 0·05 on either side, is higher than would be expected from the refractivity. A cleavage flake exhibits in convergent polarized light a wide-angled biaxial picture, the ‘eyes’ lying outside the field of view. The relation of the principal optical directions and the directions of single refraction to the crystal are shown in Fig. 27. The hardness is 8 on Mohs’s scale, and in this character it is surpassed only by chrysoberyl, corundum, and diamond. Topaz is pyro-electric, in which respect tourmaline alone exceeds it, and it may be strongly electrified by friction. Although the range of refraction overlaps that of tourmaline, there is no risk of confusion, because the latter has nearly thrice the amount of double refraction (cf. p. 29). Apart from the difference in refraction, a yellow topaz ought never to be confused with a yellow quartz, because the former sinks, and the latter floats in methylene iodide. The same test distinguishes topaz from beryl, and, indeed, from tourmaline also. Judged by the criterion of price, topaz is not in the first rank of precious stones. Stones of good colour and free from flaws are now, however, scarce. Pale stones are worth very little, possibly less than 4s. a carat, but the price rapidly advances with increase in colour, reaching 20s. for yellow, 80s. for pink and blue stones. Since topazes are procurable in all sizes customary in jewellery, the rates vary but slightly, if at all, with the size.