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Opal is not crystalline in the usual sense. It is made of amorphous spheres of silica (SiO2) packed closely together. The sphere packing only mimics the extremely ordered arrangement of the much smaller single atoms and ions in the lattice of a true crystal.
Opal sphere lattices are far less ordered. The spheres are of varied size and thus they do not pack together well. Take a box of uniform sized marbles and rattle it well too and fro – the marbles will arrange into a close packed array. Add a few marbles of different size and this no longer happens, there will only be small groups, if any, regularly arranged. The out of size marbles disrupt the close packing.
Precious or gem opal, sought for its flashes of colour, has spheres 150 – 300 nm diameter. In comparison red light has a wavelength of ~600nm. Precious opal has macro sized domains where the spheres are fairly uniform in size and, locally at least, pack into a fairly orderly tetragonal, cubic or hexagonal lattice. These are the domains that can flash colour.