Probably irradiated rock crystal. Very clean specimen; angles [skipping after 45 in the crown] as in ‘Opposing Corners 15’ by Robert H. Long, excellent cut by Tan Pen.
A yellow to yellow-orange or yellow-green variety of quartz. Natural citrine is very rare. Large quantities of amethyst, usually of lesser quality, are heated to turn it yellow or orange and sold as ‘citrine’.