Alchemy reagent \"Rebis\" (The Witcher 3: Wild Hunt)

Sobsno subject: Where to get? Formula or piece. Needed to craft a sword, climbed alchemists and herbalists in all locations and did not find it.

UPD: Rebis found. In the east (right) of the central island of Skellige, the quest is with a druid-alchemist just north (above) of the Gedineith Oak. Where does the coronation of the king / queen take place according to the plot on Skellige. Marked as an alchemist on the map.

There are no screenshots, look for your hands =)