▁is ▁the ▁f err ous ▁iron ▁end ▁member ▁of ▁the ▁class ▁of ▁gar net ▁min er als ▁representing ▁an ▁important ▁group ▁of ▁rock - form ing ▁sil ic ates , ▁which ▁are ▁the ▁main ▁constitu ents ▁of ▁the ▁Earth ' s ▁cr ust , ▁upper ▁mant le ▁and ▁transition ▁zone . ▁Al m and ine ▁cry st all izes ▁in ▁the ▁cub ic ▁space ▁group ▁I ad , ▁with ▁unit - cell ▁parameter ▁a ▁ ≈ ▁ 1 1 . 5 1 2 ▁Å ▁at ▁ 1 0 0 ▁K . ▁ ▁Al m and ine ▁is ▁ant ifer romagnet ic ▁with ▁the ▁Né el ▁temperature ▁of ▁ 7 . 5 ▁K . ▁It ▁contains ▁two ▁equivalent ▁magnetic ▁sub l att ices . ▁ ▁Occ urrence ▁Al m and ine ▁occurs ▁rather ▁abund antly ▁in ▁the ▁gem - gra vel s ▁of ▁Sri ▁L anka , ▁wh ence ▁it ▁has ▁sometimes ▁been ▁called ▁C ey lon - ruby . ▁When ▁the ▁color ▁incl ines ▁to ▁a ▁vio let ▁t int , ▁the ▁stone ▁is ▁often ▁called ▁Sy ri am ▁gar net , ▁a ▁name ▁said ▁to ▁be ▁taken ▁from ▁Sy ri am , ▁an ▁ancient ▁town ▁of ▁P egu ▁( now ▁part ▁of ▁My an mar ). ▁Lar ge ▁depos its ▁of ▁fine ▁al m and ine - gar net s ▁were ▁found , ▁some ▁years ▁ago , ▁in ▁the ▁Northern ▁Territ ory ▁of ▁Australia , ▁and ▁were ▁at ▁first ▁taken ▁for ▁rub ies ▁and ▁thus ▁they ▁were ▁known ▁in ▁trade ▁for ▁some