Hyalite Opal Identification Guide
A practical field guide to recognizing glassy, water-clear hyalite opal by its botryoidal crusts and brilliant green UV fluorescence.
Read the full Hyalite Opal encyclopedia entry →
What Hyalite Opal Looks Like
Hyalite is a colorless, water-clear variety of common opal (hydrated silica, SiO2·nH2O) that forms as glassy, globular crusts rather than crystals. Look for:
- Color: colorless to faintly milky, sometimes a faint blue or yellow tint
- Luster: vitreous (glassy), often wet-looking
- Transparency: transparent to translucent
- Habit/form: botryoidal (grape-like), globular, reniform crusts and droplets on a host rock; never euhedral crystals
Its nickname "Müller's glass" reflects how much it resembles drops of clear glass or hardened water spilled across a matrix.
Step-by-Step Field ID Checklist
- Confirm the form. Hyalite appears as rounded, bubbly, droplet-like coatings. If you see flat crystal faces, it is not opal.
- Check clarity. It should be glass-clear to slightly cloudy with a glossy surface.
- Test hardness. Around Mohs 5.5–6.5; it will scratch glass faintly or not at all and is scratched by quartz.
- Use a UV light. This is the single best test (see below).
- Look at the matrix. Hyalite typically sits on rhyolite, basalt, or other volcanic host rock.
Key Diagnostic Tests
- UV fluorescence: Hyalite famously glows bright green under shortwave and longwave UV due to trace uranium (uranyl ions). A vivid green glow is nearly diagnostic.
- Hardness: 5.5–6.5; softer than quartz (7).
- Streak: white.
- Fracture: conchoidal; no cleavage.
- Density: ~1.9–2.2 g/cm3, noticeably light for a glassy mineral.
- No effervescence in dilute acid (separates it from carbonate crusts).
Common Look-Alikes
- Clear glass / man-made droplets: glass feels harder, but the green UV glow settles it.
- Quartz/chalcedony botryoids: quartz is harder (7), does not fluoresce green, and chalcedony is waxier and more translucent-white.
- Precious opal: shows play-of-color flashes; hyalite does not.
- Calcite or aragonite crusts: fizz in acid; hyalite does not.
- Datolite/prehnite globs: harder and usually colored; lack the strong uranyl-green fluorescence.
Where It Is Found
Hyalite occurs in volcanic settings worldwide. Classic localities include Zacatecas and San Luis Potosí in Mexico (gem-quality green-fluorescing crusts), Valec in the Czech Republic, Hungary, and various US Western volcanic fields. It lines cavities and coats fracture surfaces in felsic to mafic volcanic rocks.
Frequently asked questions
How can you tell if it's real hyalite opal?
Genuine hyalite is glass-clear with a botryoidal, droplet-like form, has a hardness around 5.5–6.5, shows no play-of-color, and glows bright green under UV light from trace uranium. The green fluorescence plus glassy bubbly habit is essentially diagnostic.
Why does hyalite opal glow green under UV light?
It contains trace uranium as uranyl ions, which fluoresce a vivid green under both shortwave and longwave ultraviolet light. The glow is strong enough to identify hyalite in the dark.
Is hyalite opal radioactive?
It contains only trace uranium, so most specimens are at most very weakly radioactive and considered safe to handle. The uranium content is what produces the characteristic green fluorescence.
What does hyalite opal look like?
It looks like clear, glassy droplets or grape-like (botryoidal) crusts of hardened water or glass spread over a volcanic rock, transparent to slightly milky with a wet vitreous sheen.