A gemstone’s quality has three basic factors- beauty, durability and rarity.

Beauty is first, color being 50% of its value, clarity 30% and cut 20%.  Also to be considered are: proportion, symmetry and polish.  These can result in a stone’s beauty being enhanced with brilliance and dispersion.  A large percentage of colored gemstones are cut in India, notorious for retaining weight by leaving thick bottoms, consequently a dull looking stone.  Those that are of fine quality are generally re-cut after exportation.
Durability matters.  A stone need not be indestructible, but it must withstand ordinary wear.
Rarity frequently determines price.  Demand is based on rarity and fashion.  Amber was once very fashionable and the second most important import to USA.  Then it  lay dormant for decades until Jurassic Park brought it back to the world’s attention.