| eRocks Mineral Auctions
Jun 07, 2018 · This forms vesicular basalt. The giant vesicles are vertical in nature; 3m wide by 6m tall. Much later mineralisation by silica and manganese rich hydrothermal fluids produced the "amethyst hedrals," specimens of which are seen in most rock shops. The Northern England giant cavities in basalt are not formed by outgassing from the magma ...