Found this info on Trove in an 1890 news article, sorry didn't save the URL:
Diamond Mining in N. S. Wales.
AN IMPORTANT REPORT.
The Prospecting Board has forwarded to the
Minister for Mines and Agriculture the follow
ing important report : We have the honor to report
that we returned to Sydney yesterday (16th instant)
after inspecting certain sites in the Berrima and Braid
wood districts. At Berrima application has been
made for aid to prospect the gem-bearing gravels near
the source of the Wingecaribee River for diamonds
and other gems. At the Mittagong Diamond Mine, on
the watershed of the Nepean River, the diamonds
occur in a small isolated patch of well water-worn
gravel associated with a little alluvial gold, as
well as zircons and sapphires. The diamonds found
here are of a good color, and show remarkably sharp
angles and edges, from which it may be inferred that
their source cannot be very far distant. Mr. C. S.
Wilkinson, in his recent report on the mineral re
sources of the Mittagong district, states that he is in
clined to the opinion that those diamonds may have
had their origin in a mass of dark volcanic rock inter
mixed with carbonaceous material, which has burst
up through the underlying coal measures into the
Hawkesbury sandstone, and now forms part of
the actual floor on which the diamondiferous
gravel reposes. With a view to determin
ing whether or not this dark volcanic rock
is the matrix of those diamonds, aid has
already been granted out of the prospecting vote to
Dunstan and party to sink a shaft on this volcanic
rock : but of late further prospecting has been sus
pended, on account of the quantity of water lying in
the surrounding allu--als which finds its way into
the shaft. Another theory as to the possible origin
of these diamonds is that they may have been formed
by the intense metamorphic action superinduced
upon the coal measures by the intrusive syenite of
the " Gib Rock." The waters draining off this hill
fall on one side into the Nepean, and on the other
into the Wingecaribee River. Near the source of the
latter, on the property of the Hon. J. Lackey,
M.L.C., at Bong Bong, near Moss Vale, is
an extensive patch of well wator-worn ter
tiary gravel, to which Mr. C. S. Wilkinson
called attention in his above report. Mr. Wiltshire,
the warden of the district, who has taken a great
interest in the local gem gravels, has proved by
practical tests with the prospecting dish that gems
such as zircons, sapphires, and garnets occur in some
abundance at this and several other localities
between Berrima and the sources of the Wingecaribee
River. As the discovery of diamonds on the water
shed of the latter river would be of great importance,
we have already recognised that prospectors should
be employed to wash for diamonds in this neigh
borhood under the general direction of the
warden.
