While sapphires and rubies have a similar grading system, it is a bit different. When assessing the value of a colored gem, the color and clarity of the gemstone are often more important in determining value than their weight.
Colored gemstones are graded based on the four Cs: color, carat, clarity, and cut, just like diamonds. However, colored gemstones are also graded based on T and O, which stands for treatment and origin.
When assessing colored gemstones, it is important to look for uniform colors all around the gemstone. Stronger, brighter, more consistent colors are more valuable. Darker or muted colors have less value.
Carat is how much the gem weighs. Usually, the bigger the stone, the more it's worth. However, smaller ones can be more valuable if they have exceptional color, cut, and clarity.
On a diamond, the number of facets it has after being cut determines its value. When assessing rubies, sapphires, and emeralds, that is not the case. Having too many facets can actually decrease the value of a colored gem.
Clarity is judged by looking for impurities, called inclusions, in the gemstone. Bubbles, crystals, liquids, cloudiness, cracks, and anything that gets trapped in the stone are considered to be an inclusion.
Origin is where the stones are found. Specific geographic locations yield more valuable gemstones than other ones. It is a vital part of assessing value in rubies, emeralds, and sapphires. In all diamonds except pink Argyle diamonds, location isn't a significant factor in determining value.
Both rubies and sapphires are from the same mineral, corundum. However, rubies are significantly rarer. It is this rarity that makes them more expensive.