The benitoite is obtained by breaking open masses of vein rock and carefully chiseling or working the crystals out of the inclosing natrolite. Many gems are injured or ruined by this method. The removal of the natrolite by acid has been tried with partial success. Large slabs of rock 2 to 3 or more feet across are obtained coated with natrolite and carrying benitoite and neptunite. The last two minerals are either visible on the drusy surface of the natrolite or are completely covered by natrolite. The position of the benitoite and neptunite is often marked by lumps or a thickening of the natrolite crust. By carefully cutting into these lumps beautiful crystals are sometimes uncovered. Often the inclosing crust or shell of white natrolite can be split from a crystal of neptunite or benitoite in two or three large pieces, so that the covering can readily be replaced over the crystal. Such material makes beautiful specimens. Slabs of bluish hornblende rock with a drusy pure white crust of natrolite containing brilliant reddish-black neptunite and blue benitoite in fine crystals are excellent for the same purpose.
The minerals associated with benitoite are described and analyses are given in the paper of Louderback and Blasdale. Neptunite is titanium silicate containing iron, manganese, potassium, sodium, and magnesium. It occurs in black to reddish- black prismatic crystals of the monoclinic system, the length commonly being several times the thickness. It has a prismatic cleavage and the thin splinters or powder show a deep reddish-brown color. The hardness is between 5 and 6 and the specific gravity 3.18 to 3.19. Neptunite is practically insoluble in hydrochloric acid.
The natrolite, with which the benitoite and neptunite are associated, does not generally occur in distinct crystals of any size. It forms massive granular white aggregates of crystallized material with curved ridge-like or cockscomb-like groups of crystals and drusy botryoidal masses in the cavities. Natrolite is a hydrous silicate of sodium and aluminum crystallizing in the orthorhombic system.
Other minerals occurring in smaller quantity in the cavities are emerald-green copper stain, amphibole needles, albite, aegirine, and psilomelane. The amphiboles are actinolite, a variety intermediate between crossite and crocidolite, and a little glaucophane.