Weapon stones seem to be most often found in the shape of a dagger or a sword, although finding one in another shape is of course possible. Upon grasping a weapon stone, the holder is immediately aware of its true nature--it can take the form of any melee weapon the holder desires. A weapon stone may also be ordered to take the shape of a small, metallic stone (appearing as perhaps a bit of hematite or iron ore), 1/2 inch to 1 inch in diameter. 

Although the weapon stone is a magical item, weapons it manifests do not count as magical for purposes of damage resistances or immunity to nonmagical attacks. These weapons  should be counted as a normal weapon for combat purposes, including possessing the normal traits of the manifested weapon (e.g. heavy, finesse, etc.). They should be treated as magical for all other purposes. There are stories, however, of even rarer weapon stones that do transform into weapons.bearing magical bonuses (+1, +2, etc.), as well as more limited stones that only turn into one weapon (a stone that only turns into a sword, e.g.).

Manifesting a weapon from a weapon stone in stone form does not require an action. Transforming the manifested weapon into a different weapon does require an action. A weapon stone does not confer on the wielder any proficiency with the weapon so created.

Notes: Combat, Shapechanging