its fashionable prolly
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.