THE BIG 3
Emeralds, Sapphires, and Rubies (the “big 3”) are generally considered precious gemstones where as most other gemstones are considered semi-precious. ( apart from some Spinels). There are several reasons for this.
RARITY : Emeralds, Sapphires and Rubies are the rarest of all gems and therefore usually the most expensive.
DURABILITY : Another factor that plays into the cost is their durability. Sapphires and rubies are among the hardest gemstones, ranking 9 out of 10 on Mohs hardness scale.
BEAUTY : A really fine Ruby, Sapphire or Emerald can actually be valued higher per-carat than a diamond, which automatically categories these 3 as the ultimate-colored gemstones. However, the general rule is that the big 3 are the most expensive and valuable gemstones.