There is no universally accepted grading system for any gemstone other than white (colourless) diamond. Gemstones are categorized into two, precious and semiprecious. We look primarily into diamond, ruby sapphire,emerald in the list of precious, these categories are based mainly on fashion and the composition of these two lists has changed frequently over time.
Many gemstones are used in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones. Gems prices can fluctuate heavily (such as those of tanzanite over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices. Typically prices can range from 5USD/carat for a normal amethyst to 20,000-50,000USD for a collector's three carat pidgeon-blood almost "perfect" ruby.