Known and appreciated since antiquity, opal was considered a talisman by the Romans.
Emperor Napoleon I gave his wife Josephine an opal called "the fire of Troy" because of its very pronounced play of colors.

From a chemical and crystallographic point of view, it is a hydrated or gelatinous silica, milky white or bluish, with iridescent reflections.
Opal has a hardness and specific weight slightly lower 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. This is why it must be protected from heat or intense light, which can evaporate this water and cause fractures.
Ultrasound, acids or strong solvents should also be avoided.