Dealers have crystals in all sizes and colors from many localities. Matrix specimens, or crystals associated with other crystallized minerals are particularly desirable. Crystals showing color zoning are also highly collectible. Classic localities to look for include Elba, Italy, and the famous old pegmatites of Maine. Species of this group crystallize in the Hexagonal system, Trigonal Division.
Crystals are often triangular in cross-section.
Occasionally pleochroic or chatoyant. Strongly piezoelectric and pyroelectric. Insoluble in acids. Very common in pegmatites and greisen, occasionally in marbles. Na Fe Al borosilicate - Dark brown to black crystals, in divergent sprays, to 4 cm long.