Black opaque gem minerals associated with corundum in the alluvial deposits of Thailand
Black opaque gem minerals (spinel, pyroxene and magnetite) occur in gravels associated with corundom in the alluvial deposits from Den Chai and Bo Phloi, Thailand. They were investigated for some, physical properties, chemical composition Raman characteristics and XRD patterns to clarify the misnomers being used in the gem markets and to distinguish individual black gem materials. The results indicate that the black spinel lies in the spinel-hercynite series, the black pyroxene is mostly augite and the magnetite lies in the magnetite-ulvöspinel series. The study also suggests that these minerals did not originate in the same environment as their mineral counterparts that are crystallised in basaltic rocks or as inclusions in corundum.