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Black GlassVSJet
Side-by-side comparison of physical properties, fluorescence, and crystal system — to identify by observation.
- ◎ is higher in Black Glass, giving a numerical distinguishing point.
- ◎ is higher in Black Glass, giving a numerical distinguishing point.
Note: properties whose numeric ranges do not overlap are auto-extracted. Both are measurable with a refractometer or heavy-liquid bath.
| Property | A: Black Glass | B: Jet | Verdict |
|---|---|---|---|
| Refractive index (RI) | 1.480–1.700 | 1.640–1.680 | Overlap |
| Specific gravity (SG) | 2.30–4.50 | 1.30–1.35 | A higher |
| Mohs hardness | 5.0–6.0 | 2.5–4.0 | A higher |
| Birefringence | — | — | — |
| Dispersion | — | — | — |
| Property | A: Black Glass | B: Jet |
|---|---|---|
| Crystal system | 非晶質 | 非晶質（炭素質） |
| Pleochroism | — | — |
| Chelsea filter | — | — |
| UV (long-wave) | — | — |
| UV (short-wave) | weak (yellow) | inert |
◆Black glass ('French jet,' Victorian and modern costume) is singly refractive with RI 1.50–1.65 and SG 2.4–4.5 depending on lead content — much denser than real jet (1.3–1.35) and cool to the touch. The float test in saturated brine separates them instantly; jet floats or hovers, glass sinks rapidly. Glass also shows mold marks, swirl lines, and rounded gas bubbles absent in jet.
Distinguishing tools
Tools that decide between Black Glass and Jet in practice.
Note: this page is auto-generated from published GIA / CGL reference values. Individual stones, treatments, and synthetics may fall outside these ranges. For a definitive identification, consult an accredited gem laboratory.