Purple druzy with a green botryoidal coating over the top of it.
The green is celadonite, a soft clay mineral from the mica family. It lines the gas cavities in Brazilian basalt before anything else arrives, and it is the reason so many amethyst geodes from Rio Grande do Sul have a green skin under, over or through the quartz. Botryoidal means it grew in rounded bubble-like masses rather than crystals, which is normal for the way clay minerals build up.
Worth being clear about a term you will see elsewhere. This is not green amethyst. Green amethyst means prasiolite, which is almost always amethyst that has been heated in a kiln to change its colour. Nothing here has been heated. The green is a separate mineral sitting on top of the purple, and the two grew at different times.
Keep it out of direct sunlight. Sustained UV breaks down the iron centres that produce the purple, and amethyst that fades does not come back.
Mineral: Amethyst, the purple variety of quartz (SiO₂), with celadonite
Crystal system: Trigonal
Origin: Rio Grande do Sul, Brazil
Hardness: 7 on the Mohs scale; hard, though fine druzy points chip easily
Treatment: None
Dimensions: 12.63 x 7.2 x 19 cm
Weight: 1.02 kg Note: Supplied on a bespoke gold stand
You will receive the exact piece photographed.