Boulder Opal, from Australia (REF:BO8)

Opals are formed over centuries and centuries by the action of rain removing silica particles from sandstone and carrying those particles deep underground, depositing them in fissures and cavities. As the material dries out, the microscopic silica "spheres” become packed into a tightly packed crystalline lattice forming the opals as we see them in the surrounding matrix. As light travels through this lattice formation we see the play of colours which makes opal so visually appealing.

  

Boulder Opal is found in Queensland, Australia and forms in cracks or coatings in and around ironstone/sandstone boulders.