Diamonds are classified as covalent crystals and precious gemstones.
Minerals naturally occurring in the earth usually form in crystalline structures. Scientists classify these crystals into four groups according to their physical properties. These groups are covalent, metallic, ionic and molecular crystals. When a mineral crystal forms so that its clarity, color and hardness makes it pleasing to the eye and durable enough to work with, it is classified as a gem stone. Jewelers classify them into two groups: precious and semiprecious stones. Whether a stone is classified as semiprecious or precious depends on its market value at a given point in time and its cultural significance.
Covalent Crystals
Gemologists classify minerals as covalent crystals when all the atoms in the mineral are joined with covalent bonds, meaning that the electrons in the substance are shared by two or more atoms. This type of bond causes the mineral to be extremely stable, and thus, creates very hard crystals, such as diamonds and quartz. In some cases, covalent crystals form stones that are considered very desirable and are classified as gemstones.
Metallic Crystals
Metallic crystals include all metal ores. They are shiny in color, extremely durable, malleable and have a high melting point. Examples of metallic crystals found in nature are magnesium, titanium, zinc, aluminum, calcium, nickel, copper, silver, platinum, gold, lead and iron. Metallic crystals are not generally classified as gemstones.
Ionic Crystals
When a mineral forms due to the attraction of oppositely charged ions in the mineral's structure, it is said to be an ionic crystal. Crystals with ionic bonds form from the combination of a metallic element with a nonmetallic element, resulting in a strong, but sometimes brittle mineral. Cubic zirconium is an example of a ionic crystal used in jewelry.
Molecular Crystals
Molecular crystals are very soft crystalline structures, held together by bonds created by forces of attraction, such as hydrogen bonding or van der Waals forces. Such minerals dissolve easily and have a very low melting point. As such, they are not useful as gemstones and are not classified as such.
Precious Gems
When a crystalline element is hard and pleasing to the eye, and additionally, very rare and revered by a culture, it is classified as a precious gemstone. Precious gems also usually include an imperfection that affects their color, such as the existence of another mineral like iron, titanium, malachite or manganes. Since market value plays an important part of whether or not a stone is classified as precious at a given time, the stones that qualify do vary. However, several stones that have been classified consistently as precious throughout the years are diamonds, sapphires, rubies and emeralds.
Semiprecious Gems
Semiprecious gems are those which are considered less valuable than precious stones. Over 130 stones have been classified as semiprecious at one point or another, including hematite, jade, obsidian, pyrite, tanzanite, agate, opal, turquoise and peridot.
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