It is difficult to list these characteristics in the order of importance, but _hardness_ would rank high. Hardness of a gem is best defined as its resistance to abrasion or scratching. Most commonly used for comparison is the Mohs scale, which consists of selected common minerals arranged in the order of increasing hardness. On this scale, topaz is rated as 8 in hardness, ruby as 9, and diamond, the hardest known substance, as 10. Any gem with a hardness less than that of quartz, number 7 in the scale, is unlikely to be sufficiently scratch-resistant for use as a gem. A less precise scale, using common objects for comparison, might include the fingernail with a hardness up to 2½, a copper coin up to 3, a knife blade to 5½, a piece of window glass at about 5½, and a steel file between 6 and 7, depending on the type of steel. By this scale, any stone that remains unmarred after being scraped by a piece of window glass will have a hardness greater than 5½. The more important gemstones—which include diamond, ruby, sapphire, and emerald—all have a hardness much greater than 5½. The size of a gemstone usually is indicated by its _weight_ in carats. The expression “a 10-carat stone” has meaning—if somewhat inexact—even to the nonexpert. Specifically, a carat is one-fifth of a gram, which is a unit of weight in the metric system small enough so that approximately 28 grams make an ounce. A 140-carat gemstone, then, weighs about an ounce. Another distinguishing characteristic of a gemstone is its specific gravity, which is an expression of the relationship between the stone’s own weight and the weight of an equal volume of water. We are aware of a difference in weight when we compare lead and wood, yet it would not always be correct to say that lead weighs more than wood, for a large piece of wood can weigh more than a small piece of lead. Only by comparing equal volumes of these materials can the extent of the weight difference be clear and unmistakable. Diamond is 3½ times heavier than the same volume of water, so its specific gravity is 3.5. Since each species of gem has its own specific gravity, which can be determined without harming the stone, this standard of comparison is a valuable aid in identifying gems. Several techniques have been devised for determining specific gravity, and most of them make use of some kind of weighing device or balance. Among the most striking and useful of the distinguishing characteristics of gemstones are those that involve the effects on light. An important effect of a gem on light is the production of color, upon which many gems depend for their beauty. Some gem materials, such as lapis lazuli, have little to offer except color. Many gemstones vary widely in color, owing to the presence of varying but extremely small amounts of impurities. Thus, the gemstone beryl may occur as blue-green (aquamarine), as pink (morganite), as rich green (emerald), as yellow (golden beryl), or even colorless (goshenite).