See Classical Latin paederos (Lewis and Short 1288c), from Byzantine Greek παιδέρως. As used in Pliny the word refers to a type of precious stone which can be interpreted as either an opal or amethyst. See OED peridot n., TLF péridot and FEW pierres précieuses (21,42b) for the suggestion that the etymon, through metathesis, produced the more common peridout. The DMBLS suggests the form peridotus has been borrowed from ME or AN.