There are the precious gemstones like diamond, ruby, sapphire and emerald. There are also the semi-precious gemstones like turquoise, amethyst, garnet and quartz. There are also the gemstones that are considered birthstones (like aquamarine for the month of March and peridot for the month of August) and anniversary stones (like blue topaz for the fourth year and onyx for the eleventh year).
A gemstone is a piece of mineral that is used in cut and polished form to make jewelry or other adornments. However, certain rocks or organic materials that are not minerals are also used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value.
Gemstones are classified into different groups, species and varieties. They are also characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture and luster. They may also be classified in terms of their "water." This is a recognized grading of the gem's luster, transparency and/or "brilliance." Highly transparent gems are considered "first water," while "second" or "third water" gems are those that have a lesser transparency.