No bhai its not fashion , its tradition that goes back to the Martydom of Hazrat Omer Razi AllahTaala. The persian guy named Feroz Abo Lulu *who assasinated the Great Caliph during prayers wore this karas. The killer Feroz is revered by Shias and they wear these karas to honor him. Also, you would see that the rings their Zakirs and priests wear have this gem called *" FEROZA"- A light blue gem. Feroza takes its name from Feroz Abo Lulu. The Prophet peace be upon him and his companions razi Allah Talaaa never wore such ornaments.
LOL LOL LOL
and ppl at ferozabad named the place after this guy to honor him lolz, this is probably the worst and funniest explanation for anything ever.
bro ravage have explained it very well, kara's resembles bairi's who were put to prisoners of karbala. My sunni frnds who have intrests in stones love feroza too.
I am not sure if its true but from my experience the name feroz is more common in sunni's then shias.