Sold by piece by weight. Sizes may look similar depending on shape/thickness. Each stone has been weighted. 
XS: 5g to <10g
SM: 10g to <15
MD: 15g to <20g
LG: 20g to <25g
XL: 25g to 30g
Stones cranked up by Jay Essex and Diana R. Anaya (Dianimal)