Four aspects of gemstone grading determine the per-carat value of a ruby, including color, clarity, cut and carat. Quality of a top-grade ruby means it has rare features, rather than being labelled as better than another ruby.
The best rubies occur along a spectrum consisting of a brilliant, vivid red and leading to a red with a slight purplish hue. Some people maintain that the pink gemstones are sapphires. Rubies contain inclusions, with more inclusions diminishing the transparency and brilliance of the stone, which in turn diminishes its value. The cut of the stone must optimize color while preserving as much weight as possible. Fine-quality rubies rarely occur in more than one carat in size. Ruby per-carat value increases as the size increases.