Mottled describes a surface covered with patches of different colours or shades, mixed together without any pattern — not neat spots in rows, not stripes, just uneven blotches that run into each other. Think of an old marble tabletop, a cat's grey-and-cream coat, autumn leaves turning brown in patches, the light under trees, or the walls of a damp old building where the paint has changed colour. Two things are quietly built into this word. First, the patches are irregular; if the marks were tidy and separate you would say spotted or speckled instead. Second, the colours are usually close to each other — greys and browns, greens and yellows — so the effect is soft and blurred rather than sharp. The word is a little literary; you meet it in writing and in careful description more than in everyday chat, and it often carries a faint feeling of age, damp, or illness. Skin is a common use, and there it is not a compliment: mottled skin means blotchy, and doctors use it about skin that has gone patchy and purplish because the blood is not flowing well. But about a stone, a shell, or a horse, it is simply a description, and can be beautiful.