What would be the hp and hardness of a Belt Of Stoneskin?
As far as I can tell, there is no canonical definition of the hardness and hp of most Wondrous Items, so it would be up to individual GM fiat. Sample stats for Leather objects (table can be found here) give hardness of 2 and 5hp per inch of thickness of leather. I can't imagine a belt would be more than an inch thick. Magic armour specifically adds additional bonuses based on the enhancement bonuses, but Wondrous Items don't generally have this addition.
A generous GM might also choose to allow the Stoneskin effect on the belt itself, but this is not supported in the rules as written.
Magical items are normally of fine quality, but sometimes made from an exotic variant of the base material. For example, instead of wood, you might use iron wood. instead of cow leather, you might use griffon leather, instead of alligator boots, you might use drake or dragon scales.
As YogoZuno said, 2 and 5 hit points base, however, as the belt bestows stoneskin, it is reasonable to assume it gains a damage reduction of 10. You could stack this hardness 2/- + 10/adamantium = 12, then add the 5 hit points, for a total of 17 to destroy. Alternatively, you can consider the level of wondrous item it is, in terms of an enhancement bonus. Vorpal, for instance, provides a higher than +1 enhancement bonus. For certain items, it says:
Each +1 of enhancement bonus adds 2 to the hardness of armor, a weapon, or a shield, and +10 to the item’s hit points.
So even if the belt is too skinny to have 5 hit points, we might assume its at least 0.2" thick, for at least 1 hit point, but having a magical function and more or less parallel to Armor, we could add +10 hit points per +1 and 2 damage reduction per enhancement bonus. Since stoneskin is a level 4 spell, and a vorpal effect is roughly a 9th level spell, and a +5 enhancement bonus, we can deduce if there's no direct match for hinting at what enhancement bonus a wondrous item is equivalent to, then we can default at half spell level round up. half 9 round up is +5, half 4 round up is +2.
This gives our Belt a +4 hardness (at least hardness 6) and +20 hit points (at least 21 total hit points).
If the Stoneskin Effect is up, then we add 10/Adamant Damage reduction, because spell effects that protect characters also protect their gear, following the "saving throw" theory of targeting items, fireballs, breath weapons, etc.
So, without Stoneskin up: Hardness 6, Hit Points 21 with stoneskin up: Combined Hardness 16, Hit Points 21
If the item can activate the stoneskin without the protection of the Character, 37 damage in one hit would destroy it, if we followed this logic.
if the item requires the character to die/fail their saving throw, then the object can't be targeted until the additional conditions are met.
If the item is removed, and stoneskin is not activated, the default would be 27 damage to destroy, unless the DM rules the object itself is always protected by stoneskin, whether or not it applies to the wearer, and that would again, be 37 damage total.
Fine Quality/Masterwork rules may increase the hit points of the object. In theory, an object has either a +1 enhancement to hit or a +1 enhancement to damage, for example. That's half of 2 effects, so half an enhancement bonus, would be +1 Damage Reduction and +5 hit points, but that's extrapolation on a lot of assumptions.
In any event, a worn magical item (unless it is intended to break) should never be as flimsy as its non magical equivalent.
\$\begingroup\$ ' a worn magical item (unless it is intended to break) should never be as flimsy as its non magical equivalent.' is not necesarily true. Magical weapons and armour specifically get bonuses, but...a magical staff, for instance, actually has exactly the same stats as a nonmagical weapon staff. \$\endgroup\$– YogoZunoSep 1, 2017 at 4:18
\$\begingroup\$ d20pfsrd.com/magic-items/artifacts/minor-artifacts/… A staff of magi is intended to break. \$\endgroup\$– TristianSep 1, 2017 at 5:08
\$\begingroup\$ Above and bellow are abstract positions, you never know the position your answer will be in a few days. \$\endgroup\$ Sep 1, 2017 at 15:24
\$\begingroup\$ A comment on the bonus to hp and hardness of weapons are armor; only the Enhancement bonus (+1 to +5) apply the benefit; a +2 Vorpal Longsword only has +2 Hardness and +10 HP compared to a +1 Longsword. \$\endgroup\$– IfusasoSep 1, 2017 at 16:08
\$\begingroup\$ @Tristian Sure, a Staff of magi has stuff explicitly added about what happens when it is broken, but the point I was trying to make was that all other magical staves that aren't weapons have the stats of a normal staff. They get no improvements at all for being magical. So, this tends to reinforce that magical items DON'T automatically get harder to destroy unless they are designed to do so, and really refutes your whole answer, but specifically the part where you assert ' a worn magical item (unless it is intended to break) should never be as flimsy as its non magical equivalent.' \$\endgroup\$– YogoZunoSep 1, 2017 at 23:34