The common term is “gemstone”. Gemstones can be called precious, semi-precious or gems and can be composed of minerals, crystals or organic substances. Diamonds are crystalline, but can have minerals mixed in. Malachite is primarily a mineral that is a carbonite of copper. Amber is an organic substance that becomes hard enough to cut and polish.
The usual goal is to have gemstones that are hard enough to cut, shape and polish in order to make jewelry and other decorative items.
The categories can be informally described as precious, semi precious, jewel or gem. The Gemological Institute of America describes the categories as “gemstones, colored stones and pearls”. The categories are not strictly defined in scientific terms, but some similarity in classification crosses many cultures and dates far back in time. The traditional Western classifications go back to Ancient Greece and have developed through centuries of exploration, discovery, commerce and science.
At a very basic level, hardness, rarity and clarity are major standards for deciding whether to label something as a semi precious stone or a gem, but many softer substances are considered to be valuable for decoration or jewelry.
The specialized field of gemology is responsible for modern classifications and evaluations, where advanced technical and scientific methods are used to determine quality, hardness, rarity and other factors that are important to understanding gemstones.
Modern examination points include chemical composition, crystal structure and habit or form. Gemstones can be classified into groups, species or varieties, with color or other factors determining if a specific gemstone deserves a special designation. For example, turquoise can be a “species” of rock, but Bisbee Blue turquoise is a unique member of the turquoise rock species.
The seven classic tests for classifying and identifying minerals and rocks are also used to determine the classification of gemstones. The seven tests or scales that measure and record such qualities are: refraction, hardness (this is where the Mohs scale comes into play), specific gravity, dispersion, luster, cleavage and fracture.
Enhancing or adding to each of the above tests and scales, there will always be new and more complex technologies that will help to tell more about the substances of the Earth, how those substances were formed, and why they end up in their discovery condition and location.
The terms “first, second and third water” are sometimes used to classify gemstones based on examinations of luster, brilliance and transparency.
Sometimes there are flaws or other substances that are called “inclusions”. Inclusions can be mixed into crystal, mineral, or organic substance of the gem. Certain inclusions may downgrade the gemstone’s rating while others may add value to the gemstone.
Herkimer diamonds, for example, are quartz crystals that grow under exceedingly specific conditions. Sometimes Herkimer diamonds can grow around water, giving them a watery center. In even more rare cases, the Herkimer diamond will have some anthracite suspended in the water, with the quartz crystal grown around it all.
Organic or mineral inclusions in a gemstone usually degrade the quality of gemstones that are valued for their clarity and pure crystalline nature. But diamonds, rubies and emeralds that are colored by inclusions can also have great value. The most valued crystals have complete clarity, brilliance and luster. But, a specific inclusion can add vastly more value, as with the fictional “Pink Panther” diamond that supposedly had a mineral inclusion in the exact shape of a leaping panther.
The Gemological Institute of America has established a reputation for applying science and order to the classifying, examining and evaluation of gemstones, colored stones, and pearls. The Institute is credited with many accomplishments, including creating the D-to-Z color scale and Flawless-to-I3clarity scale for diamonds (1953).
The European competitor to the GIA is HRD Antwerp, or The Diamond High Council.
Other institutes, councils and agencies exist for the purpose of assessing, studying, rating and developing technology for evaluating gemstones.
Finally, Gemselect.com and other gemstone buying sites have extensive pages that are designed to help the beginner to identify various gemstones that are called “Precious”, “Semiprecious” or “gems”.