There are many different types of minerals, but not all of them have what it takes to make the cut and move into the gemstone category. Gems have been sought after, sometimes to the point of grave consequences, for thousands of years. Just like there are many different types of minerals, there are many different types of gems. However, gemstones all tend to have certain important characteristics in common.
Like with most things, minerals that are rare are more prized and more valuable. Rarity plays a very large roll in determining which minerals receive the title of gemstone. The rarer the mineral the more likely it was to be named a gemstone back when these categories were being decided on the marketplace. There are some minerals that meet other categories of gemstone qualification, such as beauty and durability, but are too common to have earned the superlative. An example of this would be agate, which is a plentiful variety of quartz.
For a mineral to be a true gemstone it must be durable. For a mineral to be durable it must resist scratching from abrasive materials. If a mineral is easily damaged or falls apart without much stress it won't make for a very valuable substance as it can't be trusted to stay in quality condition. For a mineral to qualify for gem status it should have a rating on the Moh's hardness scale of five or higher. This ensures that the piece will hold up over time and retain its beauty and value. Fluorite is an example of a mineral that may otherwise be qualified as a gemstone if not for its softness: it rates a four on the Moh's scale.
Beauty may be in the eye of the beholder, but when it comes to gems there are certain characteristics that help grade the level of beauty present in the stone. One key factor is the gemstone's index of refraction. The higher the level of light refraction the more the gemstone will sparkle. Other key factors that play a role in determining a gemstone's beauty are color, cut, cleavage (the flatness and symmetry of the sides) and general lack of imperfections. There is a more subjective side to this distinction as well. Those minerals that humankind generally found more aesthetically pleasing than others had a leg up in being named a gemstone.
Gemstones have a higher market value than basic minerals. The reason for this has to do with the above outlined qualities, which make them more attractive to potential owners. People wish to own stones that are more beautiful, durable and rare than not, and this drives up the overall value of these particular minerals. The four basic factors that go into deciding the value of a gemstone are known as the 4 C's: cut, carat, color and clarity. The higher a gemstone scores in these four categories the more it will be worth. Minerals that do not qualify as gemstones tend to score lower in these categories than do gemstones.
There are many different types of gemstones. Some of the most common types are diamonds, moldavite, amethyst, beryl, opal and amber. Some of these (opal, amber and moldavite) are actually not true minerals. They are instead something known as a mineraloid, but are still considered gemstones. These gemstones have different levels of hardness, beauty, value and rarity.