Many of the opals mined in Ethiopia, especially from the Welo deposits, are hydrophane opals. “Hydrophane” is a name used for a porous opal that has the ability to absorb water. A change in color or transparency often accompanies the water absorption. These opals typically have a lower specific gravity than other types of opal because of their porosity. Some of these opals can absorb enough water to produce a weight gain of up to 15%.

The hydrophane property of Ethiopian opals sometimes causes durability issues. The absorption of water can result in cracking. Because of that, owners of hydrophane opal should avoid immersing it in water. They do not absorb water instantly. It may take several hours for the opal to absorb a significant amount of water. Hydrophane opal will dewater if allowed to dry, and the dewatering can occur in a few days to a few weeks. After dewatering, the opal will have the same appearance and properties as before the water was absorbed.

The hydrophane properties of these opals are important considerations when examining and purchasing rough or cut opals. Their stability, appearance and weight can all be altered by immersion in water.