Coal jet is a variety of jet formed not from compressed driftwood but from vitrain, the bright, vitreous component of banded coal. Where the more celebrated lignite jet — exemplified by the prized Whitby material from the Yorkshire coast — derives from the slow coalification of waterlogged wood over geological time, coal jet originates from a fundamentally different precursor and consequently exhibits a distinct set of physical properties. Although it was employed in Victorian mourning jewellery alongside its lignite counterpart, coal jet occupies a secondary position in both the antique trade and the gemmological literature, largely because of its greater brittleness and less predictable behaviour under the lapidary's wheel.

Formation and Composition

Coal is not a homogeneous material. Petrographers recognise four principal maceral groups — or lithotypes — within banded bituminous coal: vitrain, clarain, durain, and fusain. Vitrain is the bright, glassy band visible to the naked eye in freshly broken coal; it is composed predominantly of the maceral group vitrinite, derived from the cell-wall tissues of woody plants that were buried, compacted, and subjected to increasing temperature and pressure over millions of years. Coal jet is essentially a compact, homogeneous mass of vitrinite-rich material that has achieved sufficient coalification to take a polish and sustain carving, yet retains the conchoidal fracture characteristic of glassy, brittle solids.

Lignite jet, by contrast, forms from isolated masses of driftwood buried in marine or lacustrine sediments under conditions that inhibited bacterial decay. The wood structure is preserved at a molecular level, and the resulting material — technically a form of lignite or sub-bituminous coal — retains a degree of fibrous toughness absent in coal jet. This structural difference is the root cause of coal jet's inferior workability.

Physical and Optical Properties

Coal jet shares the deep, velvety black colour of lignite jet and is capable of accepting a high, almost resinous lustre when polished. Its refractive index falls in a similar range to other jet materials, broadly between approximately 1.64 and 1.68, though the amorphous, heterogeneous nature of carbonaceous materials makes precise measurement difficult and of limited diagnostic value. The hardness of coal jet is generally cited as slightly higher than that of lignite jet — approximately 3.5 to 4 on the Mohs scale compared with roughly 2.5 to 4 for lignite jet — a consequence of its more advanced coalification.

The most diagnostically useful property distinguishing coal jet from lignite jet is its conchoidal fracture. When struck or stressed, coal jet breaks with smooth, shell-like curved surfaces, much like obsidian or glass, whereas lignite jet tends to fracture along planes that reflect its original woody structure. This conchoidal fracture, combined with a higher degree of brittleness, makes coal jet prone to cracking during carving and to chipping in wear — a significant practical disadvantage for jewellery use.

The streak of coal jet on an unglazed porcelain tile is brown to brownish-black, a test that has long been used by dealers to distinguish jet from black glass (French jet) and vulcanite, both of which produce different streak colours or none at all. Specific gravity for coal jet typically falls between approximately 1.3 and 1.4, consistent with its carbonaceous composition.

Distinction from Lignite Jet and Other Simulants

In the antique jewellery trade, the term "jet" is applied loosely to any black ornamental material used in mourning contexts, encompassing true lignite jet, coal jet, black glass, vulcanite (hardened rubber), bog oak, and onyx. Distinguishing coal jet from lignite jet requires attention to several criteria:

• Fracture: Conchoidal in coal jet; more irregular or sub-conchoidal, sometimes showing faint fibrous structure, in lignite jet.
• Weight: Both materials are notably light relative to glass and onyx, but coal jet may feel marginally denser than fine Whitby lignite jet.
• Warmth to the touch: Both true jet varieties warm quickly in the hand, unlike glass or stone simulants, though this test is subjective.
• Surface texture: Lignite jet, particularly Whitby material, tends to show a more uniform, slightly waxy surface; coal jet may exhibit subtle banding or inclusions reflecting its coal-seam origin.
• Streak: Both yield a brown streak; this test primarily serves to exclude glass and vulcanite rather than to distinguish the two jet varieties from each other.

Advanced gemmological testing, including infrared spectroscopy, can differentiate jet varieties with greater certainty by identifying differences in the organic molecular structure of vitrinite-derived versus wood-derived carbonaceous material.

Use in Victorian Mourning Jewellery

The Victorian period, particularly the decades following the death of Prince Albert in 1861 and Queen Victoria's subsequent prolonged mourning, generated enormous demand for black jewellery. Whitby, on the North Yorkshire coast, became the pre-eminent centre of jet carving in Britain, and the industry at its peak in the 1870s employed several hundred craftsmen. However, the supply of high-quality Whitby lignite jet was finite and subject to fluctuation, and cheaper alternatives — including coal jet sourced from colliery waste and from Spanish deposits — were introduced to meet demand.

Spanish jet, much of which originates from the region around Santiago de Compostela in Galicia and has been carved there since at least the medieval period as pilgrim souvenirs, is predominantly coal jet in character, being harder and more brittle than Whitby material. Victorian importers and retailers sometimes sold Spanish coal jet alongside or in place of Whitby lignite jet, a practice that drew complaint from the Whitby trade. The inferior durability of coal jet — its tendency to crack along the conchoidal fracture planes under the stress of setting or wear — was well recognised by informed buyers of the period.

Despite its drawbacks, coal jet was carved into the full range of mourning jewellery forms: brooches, lockets, beads, bracelets, earrings, and hair ornaments. Pieces of coal jet are encountered regularly in the antique market, though they command lower prices than comparable pieces in authenticated Whitby jet.

In the Trade Today

Contemporary collectors and dealers in antique mourning jewellery are generally aware of the distinction between lignite jet and coal jet, and auction catalogues from specialist houses will often note the material where it can be determined. The premium attached to Whitby jet — the finest lignite-based material — over coal jet reflects both its superior workability and its cultural and historical cachet. Coal jet pieces in good condition, particularly those with fine carving, retain collector interest in their own right, though they are less liquid and less consistently valued than Whitby examples.

New production of jet jewellery continues in Whitby and in Asturias and Galicia in Spain. Spanish artisans working in the Santiago de Compostela tradition continue to carve coal jet into religious and secular ornaments, maintaining a craft tradition of considerable antiquity. In the broader gemstone trade, coal jet is rarely encountered outside the antique and artisan jewellery sectors; it has no significant role in contemporary fine jewellery.

Further Reading

• GIA Gem Encyclopedia: Jet
• International Gem Society: Jet