Page:Handbook of Precious Stones.djvu/134

 Alcyonaria, class Anthozoa, and sub-kingdom Cœlenterata; the rare black coral, which is entirely horny and has but a trace of earthy matter in its composition, belongs to the other sub-class of Anthozoa, namely, Zoantharia. The solid compact part of the coral animal, or polypdom, in the case of Corallium nobile, is mainly calcium carbonate (carbonate of lime), with small quantities of magnesium carbonate, iron oxide, and organic matter; the exact nature of the red colouring matter remains unknown.

Coral is mainly obtained from the Mediterranean, the coasts of Provence, Majorca, Minorca, and North Africa being the best localities. The coral grows on rocks at depths varying from 30 to 130 fathoms, but a depth of 80 fathoms is considered most favourable.

The price of coral varies much from five shillings to £120 the ounce; the pale rose-pink variety is the most esteemed.

A good series of specimens of coral was bequeathed to the Museum in 1870 by Mr. Alfred Davis; it is now in the Branch Museum at Bethnal Green.

Several beautiful minerals and other natural products not entitled to rank as precious stones have been described in the preceding pages. A limit had to be set to the expansion of this handbook, or space might have been found for notices of apatite, a calcium fluo—or chloro-phosphate with a hardness of 5 and a density of 3·2, sometimes occurring in perfectly transparent crystals of leaf -green or sky-blue hues: fluor-spar, a still softer mineral less suitable for use as a gem; and idocrase or vesuvianite, a calcium aluminium silicate much resembling epidote (page 106), and having a hardness of 6½ and a density of 3·4.