This is a large metal helm, appropriate to be worn with any sort of heavy armor.
Your appraisal of the item reveals the following: This steel helm was forged 350 years ago by a master armorer whose symbol can be seen along the inner edge of the helm. The stone is obsidian. Though it could be worn as armor, the value of this helm is as a historical artifact. You could sell it on the open market for about 35 gold crowns (15 for the helm and another 20 for the obsidian stone). To a collector who understands the significance of its crafter, though, you could potentially garner another 150 gold crowns.