Опал — свойства и значение драгоценного камня
https://Karatto.ru/dragotsennye-kamni/opal.html
dragotsennye-kamni/opal.html.
What Is Opal and What Is It Made Of | Opal Auctions
https://www.opalauctions.com/learn/opal-information/what-is-opal
Opal is a non-crystalline form of mineral silica. Opal is related to the more common crystalline The chemical composition of opal is SiO2H2O, silicon dioxide combined with water (an opal stone may...