Natural Mexican Opal Smooth Roundel Beads Strand Length 8 Inches and Size 6mm approx. Top Quality Mexican Opal ~ Transparent Beads ~ Very Rare Opal is a hydrated amorphous form of silica; its water content may range from 3% to 21% by weight, but is usually between 6% to 10%. Because of its amorphous character it is classed as a mineraloid, unlike the other crystalline forms of silica which are classed as minerals.