Opal is an amorphous variety of silicon dioxide (SiO), unlike quartz, citrine, and agate, which have a crystalline structure. Opal’s lack of a crystal structure is what makes it an amorphous form of SiO.
Group I | Group II |
P. Sillimanite | 1. First order |
Q. Quartz | 2. Second order |
R. Muscovite | 3. Greater than third order |
S. Calcite | 4. Third order variegated |