Gemstone

A gemstone—also known as a fine gem, jewel, precious stone, semiprecious stone, or simply a gem—is typically a piece of mineral crystal that is cut or polished for use in jewelry or other decorative items. Some rocks, like lapis lazuli, opal, and obsidian, along with certain non-mineral organic materials such as amber, jet, and pearl, are also commonly used in jewelry and are therefore often classified as gemstones. While most gemstones are hard, a few softer minerals, such as brazilianite, may be used in jewelry due to their color, luster, or other visually appealing physical properties. However, soft minerals are generally avoided because they tend to be brittle and lack durability. Gemstones are found worldwide. As of 2023, the colored gemstone industry (excluding diamonds) is valued at roughly US$1.55 billion, with projections showing a steady increase to $4.46 billion by 2033. A gem expert is called a gemologist, a gem maker is a lapidarist or gemcutter, and a diamond cutter is known as a diamantaire.

In Western tradition, dating back to ancient Greece, gemstones are divided into precious and semi-precious categories, a distinction also found in other cultures. Today, the precious stones are emerald, ruby, sapphire, and diamond, while all others are considered semi-precious. This classification originally reflected rarity and quality in ancient times: all four are translucent, have fine color in their purest forms (except colorless diamond), and are very hard, scoring 8 to 10 on the Mohs scale. Other stones are grouped by color, translucency, and hardness. This traditional division does not always match modern value; for instance, most garnets are inexpensive, but a green garnet called tsavorite can be far more valuable than a mid-quality emerald. In art history and archaeology, the term "hardstone" is used for semi-precious gems. Commercially, the terms "precious" and "semi-precious" can be misleading, as they imply certain stones are always more valuable, which is not always true in the market—though they often reflect general desirability.

Modern gemologists identify gemstones using technical terminology. The first characteristic considered is chemical composition: diamonds are carbon (C), while sapphires and rubies are aluminum oxide (Al₂O₃). Many gems are crystals classified by their crystal system, such as cubic, trigonal, or monoclinic. Another term is "habit," the typical form in which a gem is found; for example, diamonds, with a cubic crystal system, often appear as octahedrons. Gemstones are also grouped into classes, species, and varieties. Ruby is the red variety of the species corundum, while any other color of corundum is called sapphire. Beryl varieties include emerald (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow), and morganite (pink).

Gems are characterized by color (hue, tone, and saturation), optical phenomena, luster, refractive index, birefringence, dispersion, specific gravity, hardness, cleavage, and fracture. They may show pleochroism or double refraction, and can have luminescence or a distinctive absorption spectrum. Gemstones are also graded by their "water," a measure of luster, transparency, or brilliance: very transparent gems are "first water," while those with less transparency are "second" or "third water." Inclusions are internal material or flaws within a stone.

There is no universally accepted grading system for all gemstones. Diamonds are graded using a system developed by the Gemological Institute of America (GIA) in the early 1950s, which introduced 10x magnification as the standard for clarity grading. Historically, all gemstones were graded with the naked eye, and colored stones are still graded that way (assuming 20/20 vision). The "four Cs"—color, cut, clarity, and carats—are a mnemonic for diamond grading, and with adjustments, they apply to other gems as well. The importance of each factor varies: for diamonds, cut is the primary value determinant, followed by clarity and color. An ideally cut diamond sparkles by breaking light into rainbow colors (dispersion), splitting it into bright pieces (scintillation), and delivering it to the eye (brilliance). In rough crystalline form, a diamond does none of this; proper fashioning, called "cut," is essential. For colored gemstones, including colored diamonds, the purity and beauty of the color is the main quality factor. Physical characteristics that add value to colored stones include color, clarity (though emeralds always have inclusions), cut, and unusual optical phenomena like color zoning or asteria (star effects). Beyond the common gemstones—diamonds, rubies, sapphires, and emeralds—pearls and opal have also been considered precious in the jewelry trade. Before large deposits of amethyst were found in Brazil in the 19th century, amethyst itself was regarded as a precious stone.