The store at this site specializes in Opal yarn. I'm sure she could help. I think different varieties of opal come in different weights. The skeins I have all seem to be fingering (sock) weight.
BTW don't hang around her site very long, it could be lethal to your credit card.
Gladys in Michigan on a day when I understand why we live up here in the north. Bee-yoo-tiful.