Hubert Duprat provided caddis flies with flakes of gold and semiprecious stones to use for building their larvae cases. They usually use grains of sand, particles of mineral or plant material, or bits of fish bone or crustacean shell, but they will use whatever is nearby and suitable to incorporate into their silk-bonded cases (although some are more flexible than others in what they will use). Here is a video and more information about the project, including some remarks by critic and philosopher Christian Besson about behavior and intent.