Feroza, the Persian name for turquoise, has been set in rings for centuries. Its sky-blue colour comes from copper, and the darker veins running through many stones are called matrix.
Even, saturated blue usually costs more than pale or greenish stones. Matrix is a matter of taste: a fine, web-like pattern can be just as prized as a clean blue surface.
Because turquoise is porous, many stones are stabilised to make them harder and keep their colour. Ask whether a stone has been treated, and expect natural, untreated feroza to be priced higher.