Amethyst — specimen 0232, photo © NMNHS
Weight: 3.56 ct; size: 11.53 | 7.78 | 6.62 mm; shape: octagon; colour: medium dark violetish blue; strong; clarity: eye clean; cut: excellent; treatment: none.
Very clean specimen; angles [skipping after 45 in the crown] as in ‘Opposing Corners 15’ by Robert H. Long, excellent cut by Tan Pen.
More information from ‘Classification’
A violet variety of quartz. The colour is caused by iron impurities in the 10s to 100s parts per million range.