diamonds were formed about 2000 to 4000 million
years ago at a depth 150 to 300 km within the Earth's
crust. Under immense pressure and temperatures of 900
to 1300°C, carbon atoms combined to crystallize and grow
as diamonds within rocks called peridotite and eclogite.
Molten magma forced up by similar pressures, were forced
up through diamond bearing rock. It partly melted the
peridotites and eclogites, capturing the diamonds and
carrying them to the surface through deep and narrow fissures.
Nearer to the Earth's surface, the magma encountered ground
water resulting from a series of violent explosions. Fragmented
by explosions, the magma erupted at the Earth's surface
to form a pipe, expelling ash which was rich in diamonds.
Over millions of years the ash within the pipe solidified
to become a diamond bearing rock called tuff. (Source:
Australian Geographic, No 42, January to March, 1996)
on the pictures to see larger images)