Jet
Jet is organic, like amber and coral, but it forms from an entirely different starting material: wood, not resin. Waterlogged driftwood, buried and compressed under specific geological conditions over millions of years, becomes a dense, fine-grained form of lignite — the lowest grade of true coal — compact enough to carve and take a smooth, glassy polish rather than crumble the way ordinary coal does. That’s the practical difference that made jet a carving material and coal a fuel, even though both ultimately come from buried, compressed organic matter.
Because it’s organic and amorphous rather than crystalline, jet has no crystal system and no fixed chemical formula to record — the same reasoning that leaves those fields blank for amber, another organic material with a genuinely variable, non-crystalline composition. Its hardness runs Mohs 2.5 to 4, soft enough to be worked with hand tools historically, and its refractive index sits around 1.64 to 1.68.
Jet carries a curious historical nickname: “black amber.” The two materials share nothing chemically — jet is fossilized wood, amber is fossilized tree resin — but both develop a static electric charge when rubbed, a property vivid enough that it shaped how people grouped them together long before anyone understood why either material behaved that way. (The word “electricity” itself traces back to the Greek for amber, elektron, for the same reason.) The resemblance is only skin- and static-deep: jet is opaque and always black, where amber is translucent and ranges from pale yellow through deep orange-brown.
The best-known source is Whitby, on England’s Yorkshire coast, where jet has been carved since at least Roman Britain and became closely associated with Victorian-era mourning jewelry — its matte-to-glossy black finish suited the strict dress conventions of that period. Spain, particularly Asturias, and parts of the United States also produce gem-grade jet, though Whitby remains the material’s most historically documented source.
Jet doesn’t appear on any of the historical or modern monthly birthstone lists, or on either zodiac system, tracked on this site — it’s included here as a standalone gemstone reference entry, the same way amber is.
Gemmological properties
| Mohs hardness | 2.5–4 |
|---|---|
| Crystal system | Not recorded |
| Chemical formula | Not recorded |
| Refractive index | 1.64–1.68 |
| Color family | Black |
| Variety of | — |
| Also known as | — |
| Origins | United Kingdom, Spain, United States |
| Organic material | Yes |
Jet is fossilized wood — compressed driftwood, not coal proper — and was historically nicknamed "black amber" because, like true amber, it develops a static charge when rubbed. The two materials are otherwise unrelated, one fossilized wood and the other fossilized tree resin.
Frequently asked questions
- Is jet a type of coal?
- Related, but not the same thing. Jet is a specific, dense form of lignite — the lowest grade of coal — that forms from waterlogged driftwood compressed under conditions distinct from ordinary coal formation. It's compact and fine-grained enough to take a smooth, deep polish, which is why it was carved into jewelry rather than burned as fuel.
- Why was jet called "black amber"?
- Because of a shared party trick: both jet and true amber develop a static electric charge when rubbed. Historically, that similarity led people to lump the two together under folk names like "black amber," even though the materials are otherwise unrelated — jet is fossilized wood, amber is fossilized tree resin.
- Is jet a birthstone on any of the lists this site tracks?
- No. It doesn't appear on any of the historical, US, UK, or zodiac systems documented on this site — it's covered here as a standalone gemstone reference entry.