Stabilized turquoise 4-4.5mm round. Turquoise. Persian turquoise or Turkish stone is considered the highest priced turquoise being natural and untreated. Most of the turquoise in the market, unless mined and produced from Iran, are treated. This is because natural turquoise has a negative reaction to heat which causes the stone to fracture, shatter or discolor. Untreated turquoise can fade or discolor. Birthstone of December.