The rusticle consists of up to 35% iron compounds including iron oxides, iron carbonates, and iron hydroxides. Rusticles are found in a tube shapes of iron oxides which are vertical to one another. Rusticles are found to grow at approximately  a year and are most often found in areas of sunken hulls underwater.
The remainder of the structure is a complex community of symbiotic or mutualistic microbes including bacteria Halomonas titanicae and fungi that use the rusting metal as a source of food, causing microbial corrosion and collectively producing the mineral compounds that form the rusticle as waste products.
Rusticles have been found to most often be composed of iron, calcium, chloride, magnesium, silica, sodium, and sulfate while there are other chemical compositions of rusticles but in much smaller quantities.