An Amethyst from the classic American locality, Jackson’s Crossroads. Part of the original group of Amethyst found to identify the value of the locality, this specimen was numbered specimen 18 and still bears its old labeling on one side.
We are encountered commonly by people who express a lot of love for Amethyst, but have never encountered a specimen with “blue flash”. 
Blue flash is an effect likely caused by lithium content and differently excited iron electrons,
Some locations produce more pieces that achieve the effect than others (notoriously, Jackson’s Crossroads and Brandberg), but when you find a crystal that does it well, you can’t unsee it. You begin to look for it everywhere. 
5.8 x 5 x 4 cm.