There is no generally accepted definition for the term gem or gemstones, but they all have something special, something beautiful about them. As a rule, they are beautiful, rare, and durable. Most are minerals: natural, inorganic materials with a fixed chemical composition and regular internal structure. Most gemstones are minerals (e.g. diamond), mineral aggregates, or rock. Some are organic formations such as amber pearl, etc.
For some gemstones the sources of specialness and beauty is the color, and unusual optical phenomenon, or the shine that makes them stand out in comparison to other stones. For other sones it is the hardness or interesting inclusion that make them special. Rarity also plays a role in the classification of gems. Valued characteristics comes from cutting and polishing.