In general, within each gemstone variety, a clear, medium-tone, very intense and saturated primary color is the most preferred. Muted colors or colors between hues, which you might find very attractive, are usually less expensive. Look at the color in different kinds of light. The next most important factor affecting value is clarity, with clear transparent gemstones with no visible flaws being the most valued. Some gemstone varieties, notably emerald and red tourmaline, are very rare without inclusions of some kind so the price structure takes this into account.
Phenomenal gemstones owe their stars and eyes to inclusions.
Tiny inclusions reflecting back light put the eye in cat's-eye chrysoberyl and the star in star sapphire. Inclusions can also be a birthmark, telling us where a particular gemstone was mined. 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.