Silver and sterling silver contain 92.5 percent pure silver, and may be marked 925. The U.S. registered trademark of the grading company must accompany the grade, as with gold. How to choose colored stones Gemstones can be measured by weight, by size or by both. A carat weighs one-fifth of a gram. Each carat is made up of 100 points, meaning that a half-carat stone weighs 50 points.
Size is measured in millimeters.
When shopping for rubies, sapphires, emeralds and other colored gemstones, look for a description of the color and carat weight, Florence said. Color is tricky. It's almost impossible to represent in a photo or to accurately describe. Shoppers may also want to find out whether the gemstone has been treated, Florence cautioned. But the jeweler should inform the shopper that the stone has been treated if, as a result, it needs special handling while cleaning.