Known and appreciated since Antiquity, opal is a hydrated or gelatinous silica, milky white or bluish, with iridescent reflections.

The more iridescent the reflections (called opalescence or iridescence), the more expensive the stone. The most sought-after opals are boulder opals with a black or blue matrix.

Opal has a slightly lower hardness and specific weight than quartz: 5.5 to 6.5 on the Mohs scale.

Opal also contains a quantity of water representing between 3 and 10% of the total weight of the stone. For this reason, it must be protected from heat or intense light, which can cause this water to evaporate, resulting in fractures.