tation of rocks containing diamonds have resulted
in the development of diamond-bearing placers. The South African
diamonds were first found in stream placers, leading to a search for
their source and its ultimate discovery under a blanket of soil which
completely covered the parent rock. The proportion of diamonds now mined
from placers is very small.

The diamonds of Brazil come from placer deposits. This is the principal
source of the black diamond so largely used in diamond-drilling.

The United States produces no diamonds on a commercial scale. Small
diamonds have been found in peridotite masses in Pike County, Arkansas,
but these are of very little commercial value. A few diamonds have been
found in the glacial drift of Wisconsin and adjacent states, indicating
a possible diamond-bearing source somewhere to the north which has not
yet been located (p. 317).

Pearls are concretions of lime carbonate of organic origin, and are
found in the shells of certain species of molluscs. Their color or
luster is given by organic material or by the interior shell surface
against which the pearl is formed. The principal supply comes from the
Indian and Pacific Oceans, but some are found in the fresh water mussels
of North America, in the Caribbean, and on the western coast of Mexico
and Central America.

From the beginning of history the principal source of rubies has been
upper Burma, where the stones are found in limestone or marble near the
contact with igneous rocks, associated with high-temperature minerals.
The weathering of the rock has developed placers from which most of the
rubies are recovered. Siam is also an important producer. In the United
States rubies have been found in pegmatites in North Carolina, but these
gems are of little commercial importance.

Sapphires are of the same composition as rubies and are found in much
the same localities. Most of the sapphires of the best quality come from
Siam, where they are found in sandy clay of placer origin. In the United
States sapphires are recovered from alluvial deposits along the Missouri
River near Helena, Montana, where they are supposed to have been derived
from dikes of andesite rocks. In Fergus County, Montana, they are mined
from decomposed dikes of lamprophyre (a basic igneous rock). In North
Carolina sapphire has been found in pegmatite dikes.

The principal source of fine emeralds is in the Andes in Colombia. Their
occurrence here is in calcite veins