Along with sapphires, rubies are the second hardest substance on the Mohs scale. This is because both gemstones are corundum. Ruby is red corundum, while sapphires make up all other color variations of corundum.
The ruby is a very rare and precious gemstone. A ruby's rich color, hardness, and silky shine secure this position. Indeed, color is the most important deciding factor in a ruby's value. The clarity is not as important in this process. The most desirable color in a ruby is that of pure red with just a hint of blue. This is extremely rare, and one of the most expensive of all gemstones. Here, color saturation is the determining factor in value and cost. The most desirable light to view rubies in is daylight. Inclusions in rubies are common and do not necessary indicate a lower quality stone. Again, color is the deciding factor in quality and value.