The ring of stone works fine as a legendary item, an artifact with many protective powers.
If it is ever allowed to fall in players hands, care should be taken to adjust legend to reality, so which powers grant merely a bonus, and which ones confer complete immunity. You might also want to look at the NPC Karian Stoneheart, who was heavily influenced by the ring.
Legendary names: Everstone, (the) Stability, Life Saver etc.
This ring is shaped from a single piece of rock. It is covered with fine lines resembling nothing particular, but giving it a nice, sturdy look. Wearing it doesn’t give any special feeling.
The ring of stone fits well on an average human hand’s fingers, but it doesn’t change size to fit the wearer as some cheaper rings do. If your fingers are slimmer or too thick, it’s your problem.
While wearing the ring, the user has numerous advantages. But these can easily become hindrances, as (s)he cannot deactivate the rings powers. On the other side, it may be taken of, anytime.
First, (s)he is immune to any spells or effects or anything, that would alter his/her physical form. That means all metamorphosing spells, so turning to stone is well out of question. Further, any radical change coming from ‘inside’ simply don’t work. That means, lycanthropes are stuck in their current form, shapeshifters also, vampires cannot change etc. If you changed into someone/thing else, putting the ring on again will make the form last indefinitely (unless put off).
Second, changes in your body may be caused by other spells and effects. In a given combat round, the wearer cannot loose more than 20 HP (choose a large enough value fitting your game system), be he damaged by spells, weapons or whatever. Radical changes such as loosing a limb deserve a saving throw with a bonus. If the wearer according to your rules lost all his chances and should now die, he is entitled to one more save before leaving this world. (Obviously GM’s call.)
Warning: the same limitations apply to healing spells! (nothing can heal him beyond that amount, ressurection will be a problem)
Third, as your body, so your mind. Immunity is granted to spells changing the state of mind too much, like Sleep and Hypnosis, not to mention things like Destroy Mind. Other charms influencing you in minor ways may deserve a bonus. Illusions are treated as normal, unless they, you guess it, would cause a radical change in the wearer(may be “wounded” by illusions, but never killed). Illusions changing your outward appearence are OK, so you may become invisible.
Secondary effects: it may take months to years, but the wearer becomes influenced in more ways than expected. The subtle changes are cummulative over time. Always think “stability”.
Sudden fits of rage and mood-swings simply do not happen anymore, no berserk ability would work. It becomes harder and harder to change your mind about anything. We might imagine a human mind as a pendulum circling randomly around a certain point. As such, over the years the circles become smaller and the chance of changing that point decreases. The mind becomes “stable”. The wearer may feel drawn towards a lawful alignement, but that is a question for the player and the Game Master together.
The Stone Ring may have unknown powers with elemental forces of stone. It is unknown for a creature of this element to attack the wearer, nor does the wearer feel enmity towards such creatures.
The Ring does not force any special feelings towards it on its wearer. The wearer will in time himself recognize the value and the reliability of the Ring. Even if there are moments good to put it off, like healing, few will drop it entirely.
It is unknown if the Ring has any intelligence or wishes of its own. It simply is.