Color is the key factor. But don’t assume that the darker the color, the better the stone. Color can be too dark. Generally, the more bright and vivid the color, the better. Pure, clear, medium-tones are the most preferred.
The second most important factor affecting value is clarity. Clear transparent gemstones with no visible flaws are the most valued. There is no standardized grading system for clarity because it varies by gem variety. With colored gemstones, if the inclusion doesn’t show in the face up position, it generally doesn’t matter at all. Some varieties, notably emerald and red tourmaline, are very rare without inclusions of some kind so the price structure takes this into account.
In rare cases, inclusions can increase value. Special effects like the star in star sapphire and the eye in cat’s-eye chrysoberyl are caused by inclusions. Inclusions can also be a birthmark, proving that a gemstone is from a particular place.
Gemstones are sold by weight, not by size. Prices are calculated per carat, which is one-fifth of a gram. Some gems are denser than others so the same weight stone may be a different size. For example a one-carat emerald is bigger than a one-carat ruby. Just like diamonds, the carat weight also affects the price.
Cut & Shape
A good cut is something that may not cost more but can add or subtract a lot of beauty. A well-cut faceted gemstone reflects light back evenly across its surface area when held face up. If the stone is too deep and narrow, areas will be dark. If it is too shallow and wide, parts of the stone will be washed out and lifeless. The best way to judge cut is to look at similar gemstones next to each other.
Colored gemstones come in lots of different shapes, many more than diamonds. Choose whichever style appeals to you.
Country of origin matters in the prices of high-end ruby and sapphire but it doesn’t have to matter to you. Just know that one stone can cost more than an identical stone without confirmed origin. If you are buying a larger or premium gem then you need to think about its origin. If not, don’t worry about it other than the fact that it is kind of cool to know where a gem is from.