Rubies have been among the most valuable of gemstones for thousands of years. A perfect ruby is even more rare and difficult to find than most diamonds (with perhaps the exception of some fancy colored diamonds). The rubies you may find in jewelry will therefore, usually be small and heavily included. The clarity grading for rubies is nowhere near as strict as that for diamonds. When shopping for a ruby keep in mind that an SI diamond will be much cleaner than a SI ruby. A flawless ruby is so hard to come by, you should ask to have it certified by a lab before looking at one twice. In fact, the presence of inclusions in rubies, much like emeralds, add natural character that makes your gem unique from any other. As long as the inclusions are not so extensive as to compromise the integrity of the gem or to muddle the transparency, you should rather consider the color of a ruby as much more important. The deeper, richer, and more pure the red, the more you will be paying for the ruby.
The color of love, passion, and wisdom, rubies are the red variety of mineral corundum, the crystal form of aluminum oxide. Corundum occurs naturally in every color of the rainbow. However, when corundum is any color other than red it is called a sapphire. The hardness of ruby makes it perfect for use in jewelry. It is surpassed in hardness only by diamonds.
The origin of the tradition of birthstones is sometimes attributed to the Breastplate of Aaron, the high priest of Hebrews. The breastplate of the high priest was a ceremonial garment set with 12 gemstones, each stone representing one of the twelve tribes of Israel. The twelve gems also corresponded to the twelve months of the year, and the twelve signs of the zodiac.
Ruby is the modern birthstone for July and the stone for the star sign of Cancer.
Ruby has a Mohs scale hardness rating of 9.