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A foreign material (solid, gaseous or liquid) usually minute in size, that is enclosed within a natural mineral, but not a synthetic gemstone. An inclusion may result from: a pre-existing material that was enveloped by a growing crystal, e.g. specks of iron; a substance formed when the crystal was being formed, e.g. a gas bubble or a change after the crystal was formed, e.g. by heating which causes chamical alteration and recrystallization.
Examples of solid inclusions may be:
Examples of liquid or gaseous inclusions may be water or carbonic gas, with sometimes a bubble of gas in the liquid (a 'two-phase inclusion'), and sometimes also an included crystal (a 'three-phase inclusion').
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson