This is an Amethyst Cactus from South Africa. This piece measures approx. 51 x 18 mm: weight is 20 grams.
 
Cactus Amethyst Quartz is a purple variation of traditional Quartz crystal that can be round or hexagonal in shape and is largely covered by a layer of second generation crystals. They are considered rare, found only in the mountainous regions of South Africa. These specimens are from Mpumalanga, South Africa specifically.