Sarah Steele is a professional gemmologist, and Fellow of the Gemmological Association of Great Britain (FGA,DGA). As such, she is uniquely qualified to research jet materials.
Gemmologists currently class Whitby Jet as an organic gemstone. The Organic Group includes such gem materials as amber, coral, pearl, bone and ivory. Unlike other gemstones, these materials are not crystalline but are derived from biological processes. Whitby Jet is just one member of a much larger group of jet materials, all of which are better described as biogenic gems. Jet, as a material is not particularly rare in the geological record and Sarah’s research database includes jet from eighteen countries sourced from across five continents. What makes Whitby Jet unique, however, is its stability and durability as a gemstone, Whitby Jet, along with Asturian Jet from the North of Spain are the best quality jet materials in the world from a jewellery perspective, a property reflected in its longevity in the archaeological record. In order to better classify the Jet Group, Sarah is working on a new branch of gemmology – ‘Hydrocarbon Gemmology’ which will allow us to for the first time understand the nature of black organic materials used in the archaeological record.
“Stones have been known to move and trees to speak”
The following gemmological data refers to Whitby Jet.
|Colour ||Jet-black |
|Transparency ||Opaque using the loupe, semi-translucent to a couple of microns using a microscope |
|RI ||1.66 (distant vision) |
|Hardness ||3.5-4.0 Mohs equivalent using Vickers Micro-hardness. It is not possible to measure the hardness of coals, so although always quoted, these values must be used with caution. |
|Inclusions ||Should be clean but rarely isolated quartz grains. The microscope may reveal a ‘ghost’ tree ring structure |
|SG ||1.3 |
|Lustre ||Liquid, mirror-like, vitreous |
|Fracture ||Dull often semi- iridescent conchoidal in hard Whitby Jet. Bright conchoidal fracture in Soft Whitby Jet. |
|Streak ||Light cinnamon brown (gem quality)-chocolate brown (soft jet) |
|Thermal conductivity ||Poor |
|Stability ||Hard Whitby Jet has excellent resistance to chemical attack so stable in the archaeological record for 5000+ years. Soft Whitby jet stable for 100 years. Can tolerate heat to 100°C |
- Beware. Once polished modern simulants are indistinguishable from genuine Whitby Jet, however, the following data is useful when considering C19th jewellery.
- Intense Jet black
- Warm liquid lustre, not the cold black lustre of onyx
- Poor thermal conductivity – feels body temperature
- Low SG but feels slightly dense for most plastics
- Develops a patina, fine network of scratches with lope
- Dead flat surface free of undulations
- Should be no woody texture/structures visible with the naked eye
- Whitby Jet never draws black. Beware as modern Russian simulants have slightly lighter streak than Hard Whitby Jet. The streak is unreliable for identification unless on the beach at Whitby!
- Antique Whitby pieces are all hand carved or faceted so each unique
- Free of moulding marks
- Conchoidal fracture often visible on the edge of a drill hole