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Black onyx v argillite


G’day; as a substitute for black onyx, have you ever thought
of using argillite? Where I live it is extremely common on the
beaches, though there are deposits in the nearby mountains. It
is extremely hard; so hard that the Maoris used it to make tools
with, including knives and carving tools, though they preferred
the local jade when they could get it as it is tougher and less
brittle. But as the local beaches are strewn with pebbles,
fist sized cobbles and bigger, there is no need to go to the
ancient Maori quarry for it.

As one picks it off the beach it is a very dark grey with very
tiny crystalline flecks, and using diamond saws and burrs, and
water-cooled grindstones, it is able to be shaped into
cabochons. It is in fact a dense and strongly consolidated
siltstone, where the original silt was composed of extremely
fine particles, and has a fracture that is nearest to conchoidal.
With the usual lapidary polishing methods, finishing with tin
oxide on a leather lap, it will take a brilliant polish, then
appearing quite black. I have cut and polished cabochons and
bezel mounted them in the odd bracelet, pendant and ring, and
when one tells the recipient that it is made from stones off the
local beach, they look at one askance, as if to say, ‘you’re
pulling my leg’. Argillite is a very common stone and can be
found all over the world. Cheers,

    /\      John Burgess
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(_______)  In  temperate Mapua NZ -

Autumn’s here and winter’s close. Icy rain today, and violent wind gusts,
filling my little cascade and pond with dead leaves.