Rubies must be transparent to possess gem value. Transparent rubies of large size are even rarer than diamonds. Transparent, flawless rubies cost more than any other gemstone, except for intensely-colored fancy diamonds. Opaque or semi-opaque rubies have little value, even if they display asterism, such as for star rubies.

Ruby clarity is measured using a jeweler's loupe under 10X magnification. Rubies are considered a Type II gemstone, which include gemstones that tend to be "eye clean" but normally have a few inclusions.