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This amethyst cluster is a small part of what would have been a much larger vugh. The sample is most likely from the Agoenormous basalt fields in Brazil. Specimen is 16 cm × 13 cm × 4 cm.
Amethyst is a variety of α-quartz. The colouration is caused by minor quantities of ferryl iron (tetravalent). This state of iron is rather unstable and reverts to ferric iron upon heating. Heat treatment of amethyst will accordingly produce citrine (yellow-orange in colour), which is how similar specimens from the same field are are produced artificially.
Reprocessed model uploaded 14th Feb 2021