Cacholong Opal Identification Guide

What Cacholong Opal Looks Like

Cacholong is a variety of common opal (potch), not precious opal, so it shows no play-of-color. It is almost always opaque, milky to porcelain white, occasionally with cream, gray, or bluish tints. The luster is dull to waxy or porcelain-like, never glassy. It commonly forms smooth nodules, rinds, or porous masses, and is frequently found intergrown with or coating chalcedony. Surfaces often look matte and chalky rather than reflective.

Step-by-Step Field-ID Checklist

1. Confirm it is opaque and white with no flashes of spectral color when tilted.
2. Check the luster — cacholong is matte, waxy, or porcelain-like, distinguishing it from glassy quartz.
3. Heft it — it should feel notably light for its size.
4. Touch your tongue or a wet finger to it — true cacholong, being a porous hydrophane opal, tends to stick to the tongue and can absorb water.
5. Scratch test — it will scratch with a steel knife only with difficulty (Mohs ~5.5–6.5).
6. Look for a chalcedony backing or matrix, common in cacholong nodules.

Key Diagnostic Tests

• Mohs hardness: about 5.5–6.5 — softer than quartz (7), so quartz will scratch it.
• Streak: white.
• Fracture: conchoidal to uneven; no cleavage (it is amorphous silica, SiO2·nH2O).
• Density: low, roughly 1.9–2.2 g/cm3 — lighter than calcite, quartz, and howlite.
• Hydrophane behavior: porous cacholong sticks to a wet tongue and may turn more translucent when soaked in water, returning to white when dry. This is a strong confirmation.
• Acid: does not fizz in dilute HCl (rules out white carbonates).

Common Look-Alikes and How to Tell Them Apart

• Howlite: also chalky white but is softer (Mohs 3.5) and usually shows gray veining; cacholong is harder and typically vein-free. Howlite fizzes weakly to nothing but is much easier to scratch.
• White magnesite / white calcite: carbonates effervesce in dilute acid; cacholong does not.
• White chalcedony / milky quartz: harder (Mohs 7), heavier, with a waxier-to-glassy luster, and they do not stick to the tongue.
• Common white opal (other potch): essentially the same species; cacholong is specifically the porcelain-textured, often hydrophane white form.
• Magnesite or dyed howlite beads sold as opal: acid test and hardness separate them.

Where Cacholong Is Typically Found

Classic cacholong comes from Kazakhstan and the Mongolian/Central Asian steppe, with notable material from Iceland, Russia, India, and the western United States. It forms as a low-temperature silica precipitate in volcanic vesicles, weathered basalt, and silicified sediments, often as nodules, geode linings, or chalcedony-associated rinds.