carving basalt

[Simon Brown <moonsong@zzzzzzzzzzzzzzz>]

Hi George, I'll have a go:

Black granite is really gabbro (or anorthosite), which is made from the same
minerals as basalt (almost no silica), but formed differently.

Gabbro is plutonic (formed deep underground: hence large crystals)

Basalt is volcanic (solidified in the throat of a volcanic vent, or on the surface, or
on the sea floor: hence, no visible crystals).

Basalt can also be scoriaceous (full of small voids) depending on the carbon
dioxide content or frothiness of the magma at cooling.

Columnar basalt forms when the magma surface is chilled, and the columns
extend at right angles to the chilled surface. The brown surface is caused by
mineralisation between the columns or blocks where the water escapes from the
magma as steam.

Black granites are notoriously difficult to polish because of their low silica
content, and each type requires a slight variation in technique such as trying
different grades of abrasive.

Commercially slabbed black granite is often coated with some tricky stuff like oils
or dyes to deepen the colour. You can pick it by heating the surface with a gas
torch and wiping with a white cloth.

I have tried to polish hard, dense basalt but only achieved a greasy shine.
Basalt is difficult to carve because it has fine crystals (as well as being glassy
and tough), and tends to pluck out from the face in flakes, but it is softer than
granite. Stick to diamond saws and abrasives for shaping and detailing.

http://www.findstone.com has a vast amount of information about commercial
dimension stones (you may already be familiar with this site), check out the 2800
stone images

Regards,
Simon moonsong[AT]optusnet.com.au