Alcyonaria,
class Anthozoa, and sub-kingdom Coelenterata ; 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 
and a density of 3.4.
