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Glossary 351
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Noble. The highest type of a specified kind of gem, as " Noble Opal." A synonym of " Precious."
Nodules. A rounded irregular-shaped lump or mass, sometimes enclosing a foreign body in the centre.
Occurrence. To be found existing.
Octahedron. Two four-sided pyramids united base to base.
Off Colour. Having but a tint of desirable colour.
Old Mine. Diamonds from the old Brazilian fields; old cut diamonds of good colour.
Opacity. The quality or state of being impervious to light.
Opalescence. A milky or pearly reflection from the inÂterior of a stone.
Opalescent. Resembling or having the tints of opal; reflecting lustre from a single spot.
Opaque. When no light is transmitted.
Optic Axis. The line in a double refracting crystal in the direction of which no double refraction occurs.
Organic. Pertaining to the animal or vegetable kingdom.
Oriental. A
term much used in the gem trade to disÂtinguish stones of entirely
differing chemical and crystallographic nature to which a common name
is applied, as " Oriental topaz," bestowed on specimens of yellow
corundum of gem quality.
Original Lots. Unbroken parcels of diamonds as graded and assorted at the mines.
Orthorhombic. (Trimetric.) Having three unequal axes intersecting at right angles.
Oxide. The product of the combination of oxygen with a metal or metalloid.
Panning. Primitive process of washing gravel by placer miners in search for gems.
Pearly. Resembling the sheen of the pearl.
Percussion. (Shaking
Table.) A form of ore-separating apparatus consisting of a slightly
sloping table on which stamped ore or metalliferous sand is placed to
be sorted by gravity. A stream of water is directed over the ore, and
the table is subjected to concusssion at intervals.
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