Jet is a popular, lightweight organic gemstone that has fascinated humanity for millennia. Unlike most gems that form through mineral crystallization, jet originates from once-living material. This deeply black material has been prized since Neolithic times, reaching its peak fame during the Victorian era. It was widely adopted for mourning jewelry, driven by its somber color and Queen Victoria’s preference for it following the death of Prince Albert in 1861. This historical context ensures the material is still recognized today.
Classification and Chemical Composition
Jet is not classified as a mineral because it lacks a definitive crystalline structure and fixed chemical formula; instead, it is properly termed a mineraloid. As an organic gemstone, it belongs to the same category as amber and pearl. Specifically, jet is a form of lignite, the lowest rank of coal, but it is purer and harder than common fuel-grade lignite.
The material is essentially highly compressed, fossilized wood, most commonly originating from ancient trees belonging to the Araucariaceae family. Its bulk chemical composition is primarily carbon (about 75%), oxygen (approximately 12%), with hydrogen and sulfur constituting the remaining balance.
This high carbon content is responsible for the signature deep black color. The fossilization process results in a material hard enough to take a high, mirror-like polish, distinguishing it from less refined forms of coal. Although jet is chemically an organic hydrocarbon, its purity and structural integrity separate it from standard coal. Variations in the ratio of oxygen and sulfur are often used to distinguish between different geographical sources, such as the famous Whitby Jet.
The Unique Geological Origin
The formation of jet is a highly specific process known as “jetonisation,” requiring a rare combination of geological conditions to transform wood into the durable, lustrous material. The process begins when masses of wood are submerged and buried in fine-grained, anoxic sediments, protecting the precursor material from oxygen and rapid decay.
These sediments are often marine or estuarine clays and shales, such as the Jurassic-age Whitby Mudstone Formation in England. Over millions of years, the deeply buried wood is subjected to gradual pressure, and coalification begins. Unlike petrified wood, where the organic material is replaced by minerals, the wood material in jet is preserved as a hydrocarbon.
During this slow transformation, the wood becomes chemically impregnated with bituminous substances and organic molecules derived from the surrounding environment. This secondary impregnation aids in the preservation and hardening of the material. The optimal conditions are geographically restricted, which explains why high-quality jet is only found in a few locations worldwide. Whitby Jet, for instance, formed approximately 180 million years ago, highlighting the immense timescale required.
Identifying Physical Characteristics
One defining characteristic of jet is its remarkably low density, making it feel surprisingly lightweight when held. Its specific gravity typically ranges between 1.30 and 1.40, significantly less than most imitation materials like black onyx or glass. This low weight allowed for the creation of large, elaborate pieces of jewelry that were comfortable to wear.
Jet is relatively soft compared to many other gemstones, registering between 2.5 and 4 on the Mohs hardness scale. This softness explains why the material is easily carved and polished into intricate designs, but also means it is susceptible to scratching and damage from abrasive surfaces. When unpolished, jet possesses a dull, velvety texture, but it achieves a high, vitreous luster upon polishing.
The material’s color is typically an intense, deep black. A practical test for identification involves rubbing the material against an unglazed porcelain plate, which leaves a distinctive brown streak. Jet also exhibits a unique triboelectric property, meaning it develops a static electrical charge when rubbed against a piece of cloth.
Primary Human Uses
Jet has served primarily as an ornamental material throughout its history, with its deep black color and ease of carving making it highly desirable. The most famous period for its use was the Victorian era, where it became the preferred material for mourning jewelry. Its somber appearance matched the strict social conventions of the time.
Jewelers utilized jet to create intricate pieces, including necklaces, brooches, earrings, and lockets, often featuring delicate carving. Beyond jewelry, jet has also been used for small decorative objects, such as rosary beads and buttons. The material is broadly divided into “hard jet,” which is denser and takes a better polish, and “soft jet,” which is more brittle and prone to cracking.
Hard jet is typically sourced from marine sediments, while soft jet is often found in freshwater deposits. The superior quality of hard jet has made it the more valued variety for high-end jewelry and lasting decorative applications. Its lightweight nature and rich color continue to appeal to contemporary designers.