Ruby is the king of Gemstone
Ruby (also known as Manikyam or Manik in Hindi) is indeed one of the most beautiful and valuable mineral secrets of our earth's crust.
You ask a Gemologist and the reason they would cite will not only excite you but might also compel you to buy one. According to an American Gemologist, Ruby earns its name of "The King of Gemstones" due to the very rare vivid red hue of color in it that attracts human eyes and sooth them in a manner that they end up biding thousands of million to acquire just a single piece of rarest Rubies.
But according to the Indian Vedic astrologers and gemstone experts, the term "King of Gems" is due to the reason that Ruby represents the planet Sun (Soorya). Since Sun is considered as the Ruler of all planets in our Solar system, the Gemstone that represents the Ruler is also considered as the Ruler or King of all gemstones.
In ancient Vedic texts also, one may find Ruby mentioned as 'RatnaRaj' (means King of Gemstones in the Sanskrit language).