PRECIOUS STONES. 24 5
calcium, iron, magnesium, or less frequently manganese or chromium, and R" represents aluminium, iron, or chromium. The Garnets are thus grouped—
Aluminium Garnet I. a. Aluminium-Calcium Garnet—
Hessonite.
b. Aluminium-Magnesium Garnet
—Pyrope.
C. Aluminium-Iron Garnet —
Almandine.
d. Aluminium-Manganese Garnet
—Spessartite.
Iron Garnet II. e. Calcium-Iron Garnet—Andra-
dite.
Chromium Garnet III. F. Calcium-Chromium Garnet—
Uvarovite.
Those
Garnets which are rich in iron fuse more easily, and some of them show
magnetic properties; most garnets resist all acids except hydrofluoric.
Heat in most cases causes an alteration in colour, but the colour is
usually restored on cooling. The colour depends on the chemical
composition, and thus is uniformly distributed.
Hessonite is
also known as Essonite, Grossular, or Cinnamon-stone. The colour is
usually some shade of reddish brown, but may be yellow-brown, honey
yellow, pale green, or pale red; some specimens look more red at a
distance and more yellow when close to the eye. The specific gravity is
3-44 to 3'62, and the hardness is 7-1/2.
Ceylon
is the most important locality; in the extreme south of the island at
Belligam it is found in a gneiss, while at Matura it occurs in the
gravels. Most of the Hessonite used as a gem comes from this part.
