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Valuing a corundum gem with Black, spots or dispersed inside

Some otherwise pretty corundum gems have black in the stone. Should black greatly devalue the gem ?

It depends on the gemstone origin as well. Overall yes a black spot does effect the price by at least 50%.

And if it has more than one and combined with visible inclusions and cracks.
You can expect price to drop 70%

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If the inclusions are large and interesting, collectors may take interest, and pay for the novelty of rare/unusual inclusions. Otherwise the inclusions will greatly devalue the stone.