Rubies are graded much like any other gemstones based on clarity, color, cut and carat weight.
Traditionally the most valuable and sought after rubies on the market were of Burmese origin, known for its pure red color. Today fine Mozambican and Malagasy stones rival the quality and color of the traditional Burmese sources.
Both rubies and sapphires are in the mineral family corundum and made from aluminum oxide. Rubies that have more blue in them than red are often categorized as sapphires instead.
Large rubies with strong color and clarity are rare to find. These stones increase exponentially in value as they get larger, as long as the clarity and color are still intact. Stones with a lot of inclusions or disruptions in color are less valuable than clear, rich stones.