"A brown mineral consisting of ferrous carbonate, occurring as the main component of some kinds of ironstone or as rhombohedral crystals in mineral veins", Late 16th century (denoting lodestone): from Greek sidēros 'iron' + -ite1.
"A concretionary nodule, typically of ironstone, having radial cracks filled with calcite or another mineral", Late 18th century: modern Latin, from Latinseptum 'enclosure'.
