Sold by piece by weight. Sizes may look similar depending on shape/thickness. 
Limited inventory.
Color is more accurate in the first pictures. The ruler is measured in inches. Some sides are rougher looking. For us is part of the beauty of a natural stone that hasn't been tumbled but that may not be everyone's cup of coffee.
 
XXS: 10-14g
XS:  15-20g
SM: 22-27g
MD: 30-35g
LG: 38 -42g
XL: 45-50g
XXL:  52-57g
XXXL: 60-68g
JUMBO: 72-80g
KING SIZE: 126g