The thing is that all shields materials are equally effective as a defensive tool. Wooden shields block dragonfire just as good as candy ones. Other than quality level, you typically only care about a shields weight. High weight means more lethal bashing (usually when the dorfs weapon gets stuck/lost/loses that arm, but can randomly happen), but low weight means less encumbrance. Weakish dorfs without much stamina really need the weaker shields. Then again, weak dorf's that tire easily don't need to be apart of your front line.
A legendary shield user can already block dragonfire 99.5% of the time, so even with a higher block chance from being artifact quality.. is it really helping much? I mean it is an ARTIFACT so you can give it to your champion/militia captain/king (only good use of nobles is in the army). However, I doubt a leather shield will outperform a steel one with a highly skilled shield user. Other than for the occasional shield bash, they probably work just as well. I mean its not terrible... its just shields are weird and already a little overpowered. It'd likely kick major ass in the hands of a new recruit without high shield skill though.
Now for the really bad news. Your artifact bone hammer is effective worthless. Oh your dorfs will "oooohhhh" and "aaahhhh" over it, but anyone relying on it in actual combat will die horribly. See, artifact weapons only get a 50% higher to-hit modifer than masterwork weapons. Blunt weapons also rely heavily on their weight which makes silver warhammers as deadly (if not more so) than candy axes (due to weight of silver). This brings me to bone. Bone is one of the lighter materials in DF. See it doesn't matter how often that bone hammer hits- its only one step up from a featherwood nerf-bat. Artifact status doesn't increase its weight, damage, or armor-piercing capabilities. This is one of hte true problems of artifact weapons. Since its only a smallish to-hit bonus and nothing else, you have to get a steel/candy (or silver/plat for blunt) weapon for it to mean anything... Even bronze artifact weapons wouldn't be good enough to use over steel or candy.
Now there are only two proper uses of that bone warhammer. You can give it to your hammerer/captain of the guard. He shouldn't be able to kill anyone with it on account of the light weight. This is a good thing, as having one dorf kill another is a great cause of tantrum spirals (which lead to more beatings...). The second is decorative. You mentioned that the damned thing is expensive, so you can add it to a room's value. Putting it into a weapon trap to display in your dining room or add it to your king/noble's room. Either way the room value will skyrocket. Passing dorf's will admire it and get a warm fuzzy feeling.