  peridot' ( or ) is gem-quality olivine. olivine is a silicate mineral with formula of (mg, fe)2sio4. as peridot is the magnesium-rich variety (forsterite) the formula approaches mg2sio4. ==etymology== the origin of the name peridot is uncertain. the oxford english dictionary suggests an alteration of angloânorman pedoretÃ©s (classical latin pÃ¦derot-), a kind of opal, rather than the arabic word faridat, meaning "gem". the middle english dictionary's entry on peridot includes several variations : peridod, peritot, pelidod and pilidod &ndash; other variants substitute y for the is seen here. http://books.google.com/books?id=xr52py68wouc&lpg=pa818&dq=peridotus&pg=pa818#v=onepage&q=peridotus&f=false middle english dictionary ed kuhn part 3 the earliest use in england is in the register of the st albans abbey, in latin, and its translation in 1705 is possibly the first use of "peridot" in english. it records that on his death in 1245 bishop john bequeathed various items to the abbey: "he gave also three noble rings, in one whereof is an oriental sapphire, of wonderful bigness: in another is the stone call'd peridot, in the middle whereof is set a sapphire of great beauty; it is said to be good against the cramp, and is made almost in the form of a buckler: in the third ring is also an oriental sapphire, but less than the former."http://books.google.com/books?id=gjrnaaaamaaj&dq=%22effigies%20of%20nero%22%20cotton&pg=pa628#v=onepage&q=peridot&f=false the antiquities and history of ireland by sir james ware, 1705: cotton library folio 88 b, nero d vii ==appearance== peridot is one of the few gemstones that occur in only one color, an olive green. the intensity and tint of the green, however, depends on how much iron is contained in the crystal structure, so the color of individual peridot gems can vary from yellow--to olive--to brownish-green. the most valued color is a dark olive-green. ==occurrence== ===geologically=== olivine, of which peridot is a type, is a common mineral in mafic and [[ultramafic roc
Gemstones