This is a natural Amethyst Druse or Druzy which was sourced from Brazil and fitted to a nickel free adjustable ring.
Amethyst is one of the most well known and recognizable of crystals and has been used as a gemstone by the ancient Egyptians and was largely employed in antiquity for engraved gems and jewelry. The Greeks believed amethyst gems could prevent intoxication, while medieval European soldiers wore amethyst amulets as protection in battle in the belief that amethysts heal people and keep them cool-headed. Beads of amethyst were found in Anglo-Saxon graves in England.
Up until the 18th century, amethyst was considered one of the most valuable gemstones (along with diamond, sapphire, ruby, and emerald). However, since the discovery of extensive deposits in locations such as Brazil, it has lost most of its value.