Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%.
Opal adalah silika amorf terhidrasi (SiO2·nH2O); 3% hingga 21% dari berat keseluruhannya berupa air, namun biasanya isinya antara 6% hingga 10%.