I looked at the d20pfsrd site for serpenstone as I don't have the module.
The description under special materials didn't exactly say it is major image.
Instead the description says-
"serpentstone can be set to radiate heat or cold, or produce illusory smoke, roiling forth thin streams of scented, incense-like fumes designed to delight, enlighten, or otherwise inform serpentfolk in the vicinity. All of these effects function as major image (caster level 18th"
IOW, it gives those effects and they are treated like major image, caster level 18th.
Although there is no exact pricing in the description, the parameters (3rd level spell, caster level 18, at will?), can be used with the making magic items guidelines to determine a price for adding that to a weapon.
Note 1. From ultimate combat-
"Weapons Light and one-handed bludgeoning weapons, spears, axes, daggers and arrowheads can all be made of stone. Weapons made of stone have half the hardness of their base weapons, and have the fragile quality."
Note 2. Magic enhancements increase the hardness of weapons and there may be other methods of removing the fragile quality.
Note 3. Masterwork versions don't have the fragile quality unless the material does. So, in this case, they still would.
Note 4. The Reinforce armaments spell (1st level) removes the fragile quality, so it would be a matter of including that in the price of the weapon as an add-on like the spell-effect.