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Opal for Specimen Buyers: Welo Hydrophane, Common Opal and Doublets

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: opal is not a crystal at all but hydrous, amorphous silica (SiO2·nH2O), softer than quartz at Mohs 5.5 to 6.5. For a specimen buyer the real questions are practical: does the piece show play-of-color (precious) or not (common), will it craze as it dries, and is it solid? Ethiopian Welo opal is hydrophane: it drinks water, turns clear, loses its color flashes while wet, and recovers when dry.

Why is "opal crystal" a misnomer?

Shops list opal beside quartz points, but opal never builds a crystal lattice. It is a mineraloid: silica plus trapped water, set solid without long-range atomic order. Minerals.net's opal mineral page gives the composition as hydrous silicon dioxide, with water ranging from 3% to 21% of the total weight. That water is the key to nearly every buying risk on this page. Material that holds a lot of it can shrink and crack when it dries out; material that is porous enough can soak more of it back up.

The numbers a collector should know:

Property Value
Formula SiO2·nH2O (amorphous)
Mohs hardness 5.5 to 6.5
Specific gravity 1.98 to 2.25 (porous Ethiopian material lower)
Refractive index 1.37 to 1.47
Fluorescence Often green; varies by locality
Fracture Conchoidal, no cleavage

Hardness, gravity and refractive index are from Minerals.net's opal gemstone page. Note how light opal is. A quartz pebble the same size weighs noticeably more, which is a quick hand check against glass and resin imitations that feel denser or warmer. The mineraloids guide covers the wider family of non-crystalline "minerals".

What makes precious opal flash?

Play-of-color comes from diffraction off stacked silica spheres, and the color depends on their size. According to GIA, spheres approximately 0.1 micron across produce violet, while spheres about 0.2 microns produce red. Red therefore needs larger, rarer spheres, which is part of why GIA's grading notes that red is traditionally considered the best prominent color, orange next, then green.

Common opal has the silica but not the orderly packing, so it shows a single body color with no moving flashes. Most pink, green, blue and yellow opal sold as tumbles, rough and carvings is common opal. That is not a defect, just a different product with a different price.

Which kinds of common opal turn up as specimens?

  • Pink opal. An opaque to translucent pink common opal, usually polished as tumbles or slabs. No play-of-color is expected or claimed.
  • Green opal and prase opal. The International Gem Society describes prase opals as green to yellowish green common opals that resemble chrysoprase, and lists green prase opal from Poland colored by nickel. Tanzanian nickel-bearing opal can pass for chrysoprase, but its gravity is far lower: about 2.125 against 2.620 for chrysoprase, per the same source. The chalcedony guide covers the quartz side of that confusion.
  • Hyalite. Clear, glassy "water drop" opal coating rock. Minerals.net notes that opal often fluoresces, usually bright green, which makes hyalite a favorite under shortwave UV (see the fluorescence guide).
  • Opalized wood and fossils. Silica replacing the original organic material, keeping its shape.

What is Welo opal, and why does it change in water?

Welo (also spelled Wollo or Wello) opal comes from northern Ethiopia. A GIA Gems & Gemology study by Rondeau and colleagues reports that a new opal deposit was discovered in 2008 near the village of Wegel Tena, in volcanic rocks of Wollo Province (GIA, Gems & Gemology, Summer 2010). The same paper notes that 1,500 kg had already been extracted using rudimentary mining techniques, an unusually high yield of gem material. It also described "digit patterns", where play-of-color opal and common opal interpenetrate like fingers, a feature worth looking for in rough.

The host rock matters. Geology.com's Ethiopian opal page explains that the main vein is an opalized rhyolitic ignimbrite up to one meter thick sitting on clay; silica-bearing water pooled on the impermeable clay and silica gel filled the pores of the volcanic rock. That origin left much of the opal porous.

Hydrophane behavior, step by step

Hydrophane means "water-loving". In GIA's study, the opaque-to-translucent Wegel Tena opals became transparent when soaked in water. Here is what happens to a Welo piece left in water:

  1. Hours in. It does not happen instantly. Geology.com says it may take several hours for the opal to absorb a significant amount of water.
  2. Saturated. A milky white stone turns clear, sometimes nearly invisible, and play-of-color fades or vanishes. Some pieces gain weight of up to 15%.
  3. Drying. Back in air, the water leaves over a few days to a few weeks.
  4. Dry again. After dewatering, the opal returns to the same appearance and properties it had before.

This is why a seller who photographs Welo rough wet, or weighs it straight from a dish, is showing you something you will not get on your shelf. Ask for dry photos and a dry weight. It also means oils, perfume, dish soap and skin lotion can wick in and stain. Keep it dry.

Does Welo opal craze?

Less than its reputation suggests, depending on type. Rondeau and colleagues found the fire and crystal opals prone to breakage, while the white, opaque-to-translucent opals are remarkably durable. Ethiopia's earlier opal is a different story: Geology.com says the Shewa material that reached the market from 1994 includes much opal with crazing problems, while Welo opal is more stable.

What is crazing, and how do you spot it?

GIA defines crazing as a fine network of cracks that resembles a spider's web, caused by moisture loss through heat, excessive dryness, or exposure to bright light or direct sunlight (GIA opal quality factors). Dealers single out stones they suspect will craze, and crazed opal sells at a lower wholesale price.

For specimens and rough, three habits help:

  • Ask how long it has been out of the ground. Minerals.net notes that rough opal is usually sold in buckets of water to strengthen the play of color effect. Freshly mined pieces sold wet have not proven they can survive dry air.
  • Inspect under a loupe with raking light. Crazing shows as hair-fine lines that catch the light, often at the surface first.
  • Keep it out of windows. Display away from sun, spotlights and heaters. The humidity-sensitive minerals guide covers storage conditions.

What treatments target Ethiopian opal?

Porosity that makes Welo opal absorb water also lets it absorb treatments. Geology.com describes three:

  • Dye. Outrageous colors are obvious; subtle ones can hide, with the color concentrated near the surface.
  • Smoke. The opal is wrapped in paper and heated until the paper smolders, driving soot into the pores to make a dark body color that mimics black opal. Look for black specks along fractures under magnification.
  • Sugar and acid. The stone soaks for a few days in warm sugar water and is then submerged in sulfuric acid, leaving carbon in the pores.

IGS adds that smoke-treated opal has a low gravity (it may float in water until it absorbs enough to sink) and an unusually low refractive index of 1.38 to 1.39. A dark Ethiopian "black opal" at a bargain price should be treated as smoked until proven otherwise.

How do you tell a doublet or triplet from solid opal?

Assembled opal is real opal in a thin slice, glued to something else. GIA lists backings of obsidian, dyed black chalcedony, black glass, natural common opal, or plastic for doublets; a triplet adds a domed top of colorless quartz or clear glass. They are legitimate products when sold as assembled, but they are worth a fraction of a solid stone and should never be priced as one.

The checks:

  1. Look at the side. Geology.com shows a doublet whose side view reveals a straight "glue line" between opal and backing. A natural boulder opal has an irregular, wavy contact with its ironstone.
  2. Look from above at an angle. A triplet's dome gives a glassy, magnified look, and the colors seem to sit under the surface rather than in it.
  3. Beware closed settings. Geology.com warns that a stone in a cup bezel might be impossible to identify as solid or doublet. Loose stones and open-backed settings are easier to judge.
  4. Never soak a doublet. Water weakens the adhesive and can lift the layers.

Synthetic opal is a separate issue. IGS notes synthetics often show a cellular, scale-like or "chicken wire" pattern under magnification and do not phosphoresce.

What is opal traditionally associated with?

In crystal traditions, opal is associated with hope and creativity, and its shifting colors with change. These are beliefs, not properties of the stone.

What to look for when you buy opal specimens

Decide first whether you want color flashes or body color, then judge the piece against that goal. For precious opal, rotate it under a single light on a dark background and estimate how much of the face shows play-of-color versus dead areas. For common opal, judge evenness of color, polish and the absence of cracks. Price follows type: smooth common opal tumbles are inexpensive; precious Welo pieces are priced by brightness, pattern and size; Australian black opal sits at the top.

From our current stock:

  • Welo (Ethiopia) Opal: Wollo Province precious opal, the hydrophane material described above. Examine it dry, in a single light, and keep it away from water and oils at home.
  • Green Opal: a common opal with green body color and no play-of-color, the type IGS compares with chrysoprase. Judge it by color depth and polish, and remember it is softer than green chalcedony.
  • Pink Opal: an inexpensive pink common opal, a good way to see opal's waxy to glassy luster and light weight in the hand.

Ask any seller three things: is it precious or common, is it solid or assembled, and has it been treated? Clear answers are the best sign you are buying from someone who knows the material.

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Frequently asked questions

Is Welo opal real opal?

Yes. Welo opal is natural precious opal mined in Wollo Province, Ethiopia, since a 2008 discovery near Wegel Tena. Much of it is hydrophane, meaning it absorbs water and temporarily turns clear. That behavior is natural, not a sign of a fake, but it does make the material easy to dye or smoke-treat.

Can I put Ethiopian opal in water?

Avoid it. Hydrophane opal absorbs water over several hours, loses play-of-color while wet and can take days or weeks to dry back out. Soaking also lets dissolved dirt or soap in. Wipe it with a dry, soft cloth instead, and keep it away from oils and perfume.

What is the difference between common opal and precious opal?

Precious opal shows play-of-color, flashes of spectral color that shift as the stone moves. Common opal has the same hydrous silica chemistry but no orderly sphere packing, so it shows only its body color. Pink, green and blue opal sold as tumbles and specimens are usually common opal.

How can I tell if an opal is a doublet?

Look at the stone edge-on. A doublet shows a straight, even line where a thin opal layer meets a dark backing of obsidian, glass, chalcedony or plastic. Solid opal shows opal or natural host rock all the way through. Closed settings can hide the side, so buy loose or open-backed when possible.

Why do opals crack?

Opal holds water, and losing it too fast through heat, very dry air or strong light can produce crazing, a fine web of surface cracks. Some localities craze far more than others. Keeping specimens out of windows and away from lamps and heaters is the simplest prevention.

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