The Collaborative International Dictionary
Marcasite \Mar"ca*site\, n. [F. marcassite; cf. It. marcassita, Sp. marquesita, Pg. marquezita; all fr. Ar. marqash[=i]tha.] (Min.) A sulphide of iron resembling pyrite or common iron pyrites in composition, but differing in form; white iron pyrites.
Golden marcasite, tin. [Obs.]
Douglas Harper's Etymology Dictionary
crystalized pyrite, early 15c., from Medieval Latin marchasita, of obscure origin, perhaps via Spanish, probably from Arabic, though OED doubts this. Perhaps ultimately from Persian marquashisha [Klein]. "This name has been used for a number of substances but mainly for iron pyrites and especially for the crystalline forms used in the 18th century for ornaments." [Flood]
Wiktionary
n. (context geology English) A pale mineral, FeS2. Marcasite is physically and crystallographically distinct from pyrite, although the two have the same chemical composition.
Wikipedia
The mineral marcasite, sometimes called white iron pyrite, is iron sulfide (FeS) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide S ion having a short bonding distance between the sulfur atoms. The structures differ in how these di-anions are arranged around the Fe cations. Marcasite is lighter and more brittle than pyrite. Specimens of marcasite often crumble and break up due to the unstable crystal structure.
On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs."
In marcasite jewellery, pyrite used as a gemstone is termed "marcasite" – that is, marcasite jewellery is made from pyrite, not from the mineral marcasite. In the late medieval and early modern eras the word "marcasite" meant both pyrite and the mineral marcasite (and iron sulfides in general). The narrower, modern scientific definition for marcasite as orthorhombic iron sulfide dates from 1845. The jewellery sense for the word pre-dates this 1845 scientific redefinition. Marcasite in the scientific sense is not used as a gem due to its brittleness.
Usage examples of "marcasite".
Its coiled shell had been replaced by crystals of marcasite, which gleamed a sulfurous bronze color in the sunlight filtering through the screen.
Ansalem had collected was the shell of one of the Great Ones, changed to marcasite in the aeons before man.
I have one of the Great Ones from a time before ours, changed to marcasite but still alive in its own way.
Fragments of marcasite, eggshell thin and the color of burnished gold, fluttered up in the turbulence.
Ilna spread her kerchief and brushed the bits of marcasite from the floor with it.
Phade ran down the passage which presently joined Bird Walk, so called for the series of fabulous birds of lapis, gold, cinnabar, malachite and marcasite inlaid into the marble.
Then he noticed the way the rough grass sparkled like marcasite under the moon.
Candlelight danced on mother-of-pearl feathers edged with silver and marcasite, and flashed from tiny diamonds at the end of each feather tip.
Obviously enjoying the press of tourists and bargain hunters who swarmed the bridge over the placid Arno, he added gold chains in shimmering Italian gold, marcasite earrings, and Florentine-style brooches.
Sheer stockings, heels and marcasite earrings completed her bewitching ensemble.
And definitely no TVs, stereos, marcasite jewelry, or octagonal gold lockets.
Her black gown was finished with a little white fillet round the neck and her slippers were adorned with shameless marcasite buckles.
He was leaning on a crutch, which pushed his other shoulder up to his ears, from which marcasite rings dangled.
I tried everything: blood, hair, the soul of Saturn, marcasites, aes ustum, saffron of Mars, tincture of iron, litharge, antimony.
Your lucky stones are marcasite (or white iron pyrites if you want to drag the thing down to its lowest level) , agate (of the kind from which children's marbles are made), jasper and the more attractive emerald and topaz.