Opal in limonite collected from Coober Pedy, Australia. The opal in these specimens occurs as varying shades of blue with a vitreous lustre. Opal is actually classed as a mineraloid because it lacks crystallity i.e. it is amorphous. Being amorphous, this mineral is smooth and naturally shiny, it has not been polished and it is completely natural. Opal tends to occur in cracks of fissures within rock as evidenced by these limonite examples. Each specimen is supplied in a large ( 4 cm x 4 cm ) magnification box to enhance the viewing of the mineral. An interesting addition to any collection both for educational and display purposes, excellent for children ( +7 ) or as gift for mineral enthusiasts and those interested in geology.