London Blue Topaz Identification Guide
A practical guide to identifying London Blue Topaz by its deep steely color, high hardness, density, and look-alikes.
Read the full London Blue Topaz encyclopedia entry →
What London Blue Topaz Looks Like
London Blue Topaz is a deep, slightly grayish or "steely" blue variety of topaz, darker and more inky than sky-blue or Swiss-blue topaz. Almost all London Blue on the market is colorless natural topaz that has been irradiated and then heated to produce the stable blue.
- Color: dark, ink-blue with a faint gray or greenish undertone
- Luster: vitreous (glassy), bright on polished facets
- Transparency: transparent to translucent, usually eye-clean
- Crystal habit: orthorhombic prisms with lengthwise striations and a perfect basal cleavage; rough often shows a flat "break-off" termination
Step-by-Step Field ID Checklist
- Check the color tone. True London Blue is dark and slightly muted, not the bright cyan of Swiss blue.
- Test hardness. Topaz is Mohs 8; it scratches quartz (7) easily and resists a steel file.
- Heft it. Topaz feels noticeably heavy for its size (SG ~3.5-3.6), denser than quartz or glass.
- Look for basal cleavage. A single flat, perfect cleavage plane across the crystal is highly diagnostic for topaz.
- Examine luster and surface. Bright vitreous polish with crisp facet edges; gas bubbles or swirls suggest glass instead.
Key Diagnostic Tests
- Mohs hardness: 8 — harder than quartz, garnet, and glass.
- Streak: white.
- Cleavage: one perfect basal (pinacoidal) cleavage perpendicular to length — topaz can chip cleanly here.
- Specific gravity: 3.49-3.57, distinctly heavier than aquamarine (2.7) or glass (~2.5).
- Optics: biaxial positive, weak pleochroism.
Common Look-Alikes and How to Tell Them Apart
- Aquamarine: much lighter and lower density (SG 2.7); aquamarine is greenish-blue and lacks topaz's basal cleavage.
- Blue spinel: singly refractive (isotropic) and isometric; spinel shows no cleavage and slightly lower SG.
- Blue zircon: strong double refraction (doubled facet edges) and higher dispersion/fire; zircon is denser still.
- Glass/synthetic blue spinel imitations: glass shows bubbles, swirl lines, and is softer (scratched by quartz).
- Treated blue beryl: lighter, lower SG, hexagonal habit.
Where It Is Typically Found
London Blue is a treatment color, so the parent topaz comes from the same sources as colorless topaz: Brazil (Minas Gerais), Nigeria, Sri Lanka, Madagascar, and Russia. Most stones are irradiated in reactors and heat-treated before cutting.
Frequently asked questions
How can you tell if London Blue Topaz is real?
Real topaz is Mohs 8 (scratches quartz), heavy for its size (SG ~3.5), shows a single perfect basal cleavage, and has a bright vitreous luster. Glass imitations are softer, lighter, and show bubbles or swirl lines.
Is London Blue Topaz natural or treated?
The blue color is almost always produced by irradiating and heating naturally colorless topaz. The material itself is genuine natural topaz; the deep blue color is treatment-induced and considered stable.
London Blue Topaz vs Swiss Blue Topaz - what's the difference?
London Blue is dark, inky, and slightly gray-green, while Swiss Blue is a brighter, more saturated electric cyan. Both are treated topaz; the difference is in the irradiation/heating that controls tone.
London Blue Topaz vs aquamarine?
Aquamarine is lighter, greenish-blue, much less dense (SG 2.7 vs 3.5), hexagonal, and lacks topaz's basal cleavage. A heft test and hardness check separate them quickly.
London Blue Topaz identified by the community
Recent London Blue Topaz specimens identified with Rock Identifier.
