What Makes Blue Sapphire One of the Most Valuable Gemstones?

The most admired of colored stones, Blue Sapphire is known for its extraordinary beauty, rareness, quality, and value. Its deep blue hue as well as its remarkable durability and the long-standing relationship with royalty and exquisite jewelry has made it a sought-after gem for a long time. What makes a Blue Sapphire precious, and why are certain gemstones commanded more in comparison to other stones?