You can only strengthen armor, clothing, wearable treasures, or weapons.

Despite its plain working this ring places an inset of turquoise within a
finely twisted band of wood. The ring is small and it appears made for a
long, slender finger. Slight cuts are made into the ring, giving the edge
of the ring a way to grip the moss that lines the edges. The inset is made
of hard blue-green stone, which gives it a beautiful sea-like appearance.

A moss-lined ring, set with turquoise is a treasure, made of turquoise, and weighs 0 pounds 3 ounces.

wearable treasure!

strengthen ring
You cannot strengthen a moss-lined ring, set with turquoise.