The crystals are feebly doubly refracting, and in polarized light exhibit a banded structure parallel to the cube faces.
Crystals have the form of small, sharply defined cubes of an oliveor grass-green colour, and occur together in considerable numbers on the matrix of the specimens.
The crystals possess a perfect cleavage parallel to the plane of symmetry and are usually bladed, in habit; they are soft (H= 2), flexible and sectile.
Boron dissolves in molten aluminium, and on cooling, transparent, almost colourless crystals are obtained, possessing a lustre, hardness and refractivity near that of the diamond.
It is decomposed by water, and with a solution of yellow phosphorus in carbon bisulphide it gives a red powder of composition PBI 2, which sublimes in vacuo at 210° C. to red crystals, and when heated in a current of hydrogen loses its iodine and leaves a residue of boron phosphide PB.