Opal: where the color actually comes from, and why it can crack

Precious opal's play of color comes from a genuinely well understood physical structure, opal is made of tiny silica spheres stacked in a regular pattern, and light diffracts through the gaps between those spheres to produce the flashes of color, a real optical phenomenon rather than pigment or dye. Common opal lacks this ordered structure and shows no play of color at all. Hardness is relatively low, around 5.5 to 6.5, and opal contains a meaningful amount of water within its structure, which is why some opal can crack or craze over time if it dries out too quickly, a real and well documented risk especially for material stored in very dry, low humidity conditions or exposed to sudden temperature swings.