ll and the
deposits will probably never be important producers. During the war,
however, the United States became the largest manufacturer of thorium
nitrate and gas mantles and exported these products in considerable
quantity. An effort is now being made to secure protective legislation
against German thorium products.

GEOLOGIC FEATURES

Monazite is a mineral consisting of phosphates of cerium, lanthanum,
thorium, and other rare earths in varying proportions. The content of
thorium oxide varies from a trace up to 30 per cent, and commercial
monazite sands are usually mixed so as to bring the grade up to at least
5 per cent.

Yellowish-brown crystals of monazite have been found scattered through
granites, gneisses, and pegmatites, but in quantities ordinarily too
small to warrant mining. In general the mineral is recovered on a
commercial scale only from placers, where it has been concentrated along
with other dense, insoluble minerals such as zircon, garnet, ilmenite,
and sometimes gold. The Indian and Brazilian monazite is obtained
principally from the sands of ocean beaches, in the same localities from
which zircon is recovered (p. 189). The North and South Carolina
monazite has been obtained chiefly from stream beds, and to a slight
extent by mining and washing the rotted underlying rock, which is a
pegmatized gneiss. Monazite, together with a small amount of gold, is
also known in the stream gravels of the Boise Basin, Idaho, where a
large granitic batholith evidently carries the mineral sparsely
distributed throughout. These deposits have not been worked.

PRECIOUS STONES

ECONOMIC FEATURES

Precious stones range high in the world's annual production of mineral
values. A hundred or more minerals are used to some degree as precious
stones; but those most prized, representing upwards of 90 per cent of
the total production value, are diamond, pearl, ruby, sapphire, and
emerald. In total value the diamonds have an overwhelming dominance.
Over a ton of diamonds is mined annually.

Diamonds come mainly from South Africa, which produces over 99 per cent
of the total. Pearls come chiefly from the Indian and Pacific oceans.
Burma is the principal source of fine rubies. Siam is the principal
producer of sapphires. Colombia is the principal source of fine
emeralds.

The United States produces small amounts of sapphires (in Montana) and
pearls (from fresh-water molluscs). Diamonds, rubies, and emeralds are