Url Lanham, The insects, New York, Columbia University Press, 1964, pp. 216-218.

The best known caddis flies build portable cases of sand, small empty snail shells, or bits of vegetation bound together with silk. The shelter is highly characteristic of the species of caddis. Some are neatly coiled, like a snail shell (Fig. 63), and in fact a caddis-fly case once was described as a new species of snail. Others, found especially in stagnant water, are built of grass stems stacked like the logs of a log cabin (Fig. 64). If larvae of the sand-case type are carefully removed from their shelter, then supplied with small glass beads, they will build a new case with the beads . One entomologist found in a small Nevada stream  caddis-fly with tiny fire opals. The opals were more abundant in the sand of the cases than in the sand of the stream bottom.