The figures on the basal plane show conclusively the trigonal character, and those on the prisms the presence of the horizontal plane of symmetry. It is also evident that three planes of symmetry pass through the vertical axis and bisect the pyramidal and prismatic faces, fixing the symmetry as that of the ditrigonal- bipyramidal group, and making benitoite the type of the highest grade of trigonal symmetry.

Natural Etching - A large number of the crystals of benitoite show natural etching. The negative pyramids commonly show various degrees of dulling. This sometimes produces very striking results, especially when the crystal shows a large development of these planes with the other planes quite small, when the bulk of the surface may be very dull and even covered by a film of decomposition, in the midst of which the small faces may~ appear clear and brilliant. Triangular figures on the basal plane occasionally occur, sometimes as depressions when the triangle is in reversed position with respect to the triangle of ¼ ^ c, sometimes elevations when they are placed in the same attitude as triangle of ¼ ^ c . Occasionally hexagonal figures occur outlined by grooves parallel to the basal edges.

Hardness 61/4--61/2; distinctly above othroclase and labradorite, and below chrysolite and quartz. Density 3.64-3.67. The highest value obtained was on a flawless gemstone of moderately deep color weighing 1.53243 grams, and giving a value of 3.667. The clearest obtainable colorless fragment gave 3.65. A number of intermediate values were obtained on clear blue material and as low as 3.64. Material with flaws, cloudy white specimens, etc., in general give lower values.

Fracture, conchoidal to sub-conchoidal; cleavage very imperfect, pyramidal.

Refractive index by prism method referred to D line: w == 1.757 ; e about 1.804. Double refraction strong, and positive.

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