I thought in the comics the shield is vibranium and steel, not adamantium.
EDIT - Checking Wiki:
It says that his shield was originally said to be made of vibranium, an "experimental" iron alloy and an unknown catalyst. Seems it was later revealed the "experimental iron alloy" was "Proto-Adamanitum" which is stonger than True-Adamantium.
Further experiments with Proto-Adamanitum created True-Adamantium.
Other forms of adamanitum is "Seconday-Adamantium" which is, essentially, a cheaper knock-off of adamanitium and while is very strong can be broken/damaged.
Then there's Beta Adamantium, which occurred when it reacted with Wolverine's mutant healing powers, this change allowed the adamanitum to fuse with Wolverine's skeleton and make him nearly indestructible while the metal didn't interfere with the biological properties of bone.