Ahhh Stoneskin... I've thought I had this spell down pat over the years then looked at others interpretations of it and thought they were right and then went back to the source and wondered if there is no clear cut definition on the perhaps the most abused spell in 2nd edition AD&D.
I must admit I've played 2nd Edition Advanced Dungeons and Dragons for years and thought I knew this spell inside and out. But much like the 2nd amendment to the Constitution it's a bit puzzling in its phrasing. There is one pertinent part is unclear or at least hinges on how its interpreted. By this I mean consider the following
4th level Mage spell (page 163 in the 2nd Edition Players Handbook)
Componnents: V, S M
Casting Time: 1
Area of Effect: 1 creature
Saving Throw: None
When this spell is cast, the affected creature gains virtual immunity to any attack by cut, blow, projectile or the like. even a sword of sharpness cannot affect a creature protected by stoneskin, nor can a rock hurled by a giant, a snake's strike, etc. However, magical attacks from such spells as fireball. magic missile, lightning bolt, and so forth have their normal effects. The spell blocks 1d4 attacks, plus one attack per two levels of of experience of the caster has achieved. This limit apples regardless of attack rolls and regardless of whether the attack was physical or magical. For example, a stoneskin spell cast by a 9th-level wizard would protect against five and eight attacks. An attacking griffon would reduce the protection by three each round; four magic missiles would count as four attacks in addition to inflicting their normal damage.
The material components of the spell are granite and diamond dust sprinkled on the recipient's skin.
Now I'm no rules lawyer, but the section that says "This limit apples regardless of attack rolls and regardless of whether the attack was physical or magical."
Now you can interpret it to say: Attack rolls whether the hit or not remove one "skin" from the spell.
Or you could say it doesn't much like where the commas are in the 2nd Amendment its a bit maddening...
So what did Sage Advice have to say about it?
Stoneskin: This spell is subject to considerable abuse by player characters. Multiple stoneskins placed on a single creature are not cumulative. If two or more stoneskin spells are cast on the same creature, roll normally for the number of attacks each spell protects against. If a new spell protects against more attacks than the present spell does, the recipient gets the benefit of the increased protection; otherwise there is no effect. The caster does not necessarily know how many attacks the spell can shield him from.
Stoneskin protects only against blows, cuts, pokes, and slashes directed at the recipient. It does not protect against falls, magical attacks, touch‑delivered special attacks (such as touch‑delivered spells, energy draining, green slime, etc.), or nonmagical attacks that do not involve blows (such as flaming oil, ingested or inhaled poisons, acid, constriction, and suffocation). Stoneskin lasts for 24 hours or until the spell has absorbed its allotment of attacks.
Well this helps, but only a little.