In real life, gem appraising, cutting, polishing, faceting, etc. is a multibillion dollar industry. Not so in fiction. Not only are they much bigger, they're just naturally flawlessly cut and perfectly shiny, even while they're still in the ground, or where there's no logical way they could have been cut. Characters will find these gems studding the walls of a mine, lying close to the surface, or lying casually in open fields. Bonus points if there's also large clumps of different types, as seen in the page picture. Any place where they're abundant is usually a City of Gold.