118 PRECIOUS STONES.
lying
on Tertiary rocks, which in turn lie on very old schists; there are
also eruptive rocks associated with the Tertiary sedimentary rocks. The
minerals here associated with the Diamond are Quartz, Corundum,
Magnetite, Chromite, and some of the noble metals. Another group in the
west of Borneo, near Pontianak, is in ancient detritus, or in
conglomerates composed of rounded fragments of silicious rocks, of
schists, clay slates and volcanic rocks. The Diamonds occur usually in
sand and alluvial deposits resulting from a further disintegration of
these rocks. Associated minerals are Corundum, Magnetite, some Gold,
etc. The Diamonds themselves, though rarely of any great size, are
remarkable for their colour, especially the very rare black Diamonds
previously mentioned. The varieties Bort and Carbonado are also present.
Australia
has never produced any large stones, the heaviest being one of 1/3|
carats found in New South Wales, the part of the Commonwealth which has
so far been the most productive. Diamonds were first discovered in the
gold-washings near Bathurst in 1851 on a tributary of the Macquarie
river, along which stream ancient detrital matter has since afforded
numerous but small stones, averaging about 5-3/8 carat. This deposit is
an old river terrace, overlaid by a layer of basalt. The mineral is
accompanied by Quartz, varieties of Chalcedony, Cassiterite, Topaz,
Corundum, Brookite, Garnet, Zircon, Magnetite, Ilmenite, Tourmaline,
etc. Other deposits have been found along the upper part of the Lachlan
river. In the north of New South Wales, not far from the Queensland
border, deposits occur on the Gwydir river, a tributary of the Darling.
Here, too, the Diamonds occur in old alluvium, and
