A Rusticle is a little similar to an icicle or stalactite in appearance, but occurs under water when wrought iron rusts. They may be familiar from underwater photographs of shipwrecks.

The rusticle consists of up to 35% iron compounds including iron oxides, iron carbonates and iron hydroxides. The remainder of the structure is actually a complex community of symbiotic or mutualistic microbes including bacteria and fungi that use the rusting metal as a source of food, collectively producing the mineral compounds as waste products and hence forming the rusticle.

Since rusticles are found on wrought iron rather than other ferrous metals, and the microbes also use the sulphur and phosphorus found in the metal as impurities.

Structurally, the rusticle contains channels to allow water to flow through, and they seem to be built up in a ring structure similar to the growth rings of a tree. They are very delicate, and easily fall apart into fine powder if knocked.

The outer surface of a rusticle is smooth red in appearance from the iron (III) oxide, while the core is bright orange due to the presence of Goethite (a hydrated iron oxide) crystals.

Rusticles on the Titanic were the first investigated by Roy Cullimore, based at the University of Regina in Canada in 1996.