1.7" Quartz Rosette - Artigas, Uruguay
This is a quartz "rosette". This specimen is the tip of a stalactite projection from inside of an amethyst or quartz geode. When the geodes are mined out of hard rock, the projections often break and then the ends are cut into these gorgeous rosettes.
Silicon Dioxide, also known as SiO2 or Quartz, is the second most abundant mineral in the Earth's crust. Quartz crystals generally grow in silica-rich, hot watery solutions called hydrothermal environments, at temperatures between 100°C and 450°C, and usually under very high pressure. Quartz veins are formed when open fissures are filled with hot water during the closing stages of mountains forming, and can be hundreds of millions of years old.