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  • Scratches glass

  • Not affected by sandpaper on the bottom

  • Depending on lighting, intense light, triangles everywhere

  • Violet/light purple fluorescence

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  • $\begingroup$ What volcano (where)? What micro-geography (riverbed, road, ....)? Present in abundance or just a single object? Please edit your question. $\endgroup$ – Jan Doggen Oct 24 '17 at 12:18
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It looks likes amethyst, which is a form of purple/violet quartz, often used in jewelry.

In the Brazilian state of Minas Gerais amethyst occurs with volcanic rocks.

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  • $\begingroup$ That is an Amethyst, I've never seen anything else with quite the same colour and fracture combination. $\endgroup$ – Ash Oct 24 '17 at 11:46

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