Jet stone is a distinctive black material valued for jewelry. It is not a mineral, but an organic gemstone formed from the fossilization of ancient wood under intense pressure, similar to coal. Because of its deep black color and relatively soft nature, jet is frequently imitated by materials including glass, plastic, and various minerals. Verifying authenticity requires performing observations and simple tests that rely on its specific physical and chemical properties to distinguish it from simulants.
Initial Identification: Visual and Tactile Clues
One of the most immediate indicators of genuine jet is its unusual lightness compared to most black gemstones or glass simulants. Jet has a very low specific gravity (typically 1.30 to 1.40), meaning it feels significantly lighter than a similarly sized piece of black onyx or obsidian. Black onyx, for instance, has a density almost double that of jet.
The thermal property of jet offers another non-destructive method. Unlike mineral gemstones, which feel cold to the touch, jet is a poor thermal conductor. Genuine jet will quickly warm to room temperature or feel slightly warm against the skin, similar to plastic or wood.
Visually, genuine jet exhibits an opaque, deep black color, giving rise to the term “jet black.” When highly polished, it displays a glassy or waxy luster, lacking the internal sparkle or glassy fracture seen in obsidian or black glass. Authentic pieces often show a velvety sheen and may occasionally reveal faint remnants of the original wood grain, confirming its organic origin.
Definitive Testing: The Streak and Heat Check
More definitive identification relies on semi-destructive tests that exploit jet’s unique carbonaceous composition. The streak test involves rubbing an inconspicuous area against an unglazed ceramic surface. While many black minerals leave a black, gray, or white streak, genuine jet consistently leaves a brown streak.
This brown streak is a strong indicator of authenticity, derived from the powdered, organic components of the fossilized wood. However, some simulants, like vulcanite and bog oak, can occasionally produce a similar result.
A highly specific, yet cautious, method is the hot needle test, utilizing the thermal sensitivity of organic material. Applying a heated needle to a tiny, hidden spot will cause genuine jet to emit a faint, organic odor, often described as coal-like or sulfurous, without melting. This test should be performed with great care, as the heat can damage the piece and is not recommended for antiques or valuable items.
Jet also has a low Mohs hardness (typically 2.5 to 4.0), making it relatively soft and easily scratched compared to most minerals. This softness allows for intricate carving, but attempting to scratch it should only be done as a final, confirming step after other tests.
Distinguishing Jet from Common Imitations
Knowing the properties of simulants helps contextualize the results of the identification tests. Black glass, sometimes called “French jet,” is a common imitation that is noticeably heavier due to its higher density. Glass also feels distinctly colder to the touch and will not leave the characteristic brown streak.
Black onyx and tourmaline are much harder minerals, ranking significantly higher on the Mohs scale, and cannot be scratched easily. Black onyx, which is often dyed, is considerably denser and produces a white streak rather than a brown one.
Vulcanite and various plastics were also used historically and are lightweight like jet. When subjected to the hot needle test, vulcanite emits a strong, chemical or rubbery odor, while modern plastics typically melt and smell like burning plastic. This is a distinct difference from the coal-like scent of jet. Coal is another close relative, but it is much more brittle and leaves a black streak instead of brown.