Onyx is a crystallized stone banded with silica quartz minerals. It is a highly delicate stone, and even scratches against its surface will cause parts to flake off. Like limestone, onyx is a naturally porous stone that easily absorbs liquids. While a water stain will eventually evaporate if left on its own, other stains will damage and discolor the underlying stone. Therefore, it is highly advisable that onyx stones be sealed to protect their natural beauty against stains and scratches. They should also not be used in high traffic areas and any spills need to be immediately cleaned.