Magic items come in three general categories: those that affect the bearer, those which have an effect of their own, and those which are specifically attuned to and used by magic-users to artificially boost their power. For instance, most rings actually change the wearer’s “spirit aura” This shift is what gives the wearer the magical abilities. So, in theory, a magical non-active wearing a ring which allows the wielder access to a spell, actually just acts as an awakener, turning the wearer into a magic-user. For those who know what to look for, the astral signature of someone wearing a magic item of this type is obviously different than when they aren’t wearing it. Although the exact nature of such an item is not readily apparent, a skilled viewer can deduce the location of the device causing the difference.
The other type, generally weapons and armor, or things like that, usually have enchantments which function directly on the item, and have little effect on a wielder’s aura. These types of items can generally be detected easily as a separate aura. Though separating the auras of a warrior and his full plate-mail may be tricky.
There are also items which have both effects. In such cases it is simply both types of aura are detectable. Which may lead to the false conclusion that multiple items are at work, if one is not so skilled at aura reading.
Aura effective items change the auras of the bearer. The effects are often quite beneficial, and even lifesavers, but there is a catch. Magic items are generally made by mortals. And tampering with auras is not simple. And of course, every individual has their own unique aura. So when a magic item is affecting someone, their aura is slightly disturbed(or not so slightly, depending on the power of the item) What this means, is that effects which rely on the aura, will be slightly diffused. In other words, all those rings may be great for boosting your agility and lockpicking skill, and such, but when you miss the trap and get your fingers broken, the healer is gonna have a hell of a time fixing them with magic. Also, for the awakened, it makes it significantly harder to channel their own powers, so mages and adepts rarely carry this type of magic. The effects of this are at their worst when the item is first activated. Eventually your aura adapts, and accepts the foreign object, but not without leaving some hardened pearls around the source of irritation. Essentially, over time these penalties diminish, but will never entirely disappear. (incidentally items can be tailored to an individual’s aura, but doing so is incredibly expensive, and usually only worthwhile for items with major effects. Even still it only reduces the penalties) perhaps an item made by a divine being could overcome this limitation, but there are no trustworthy records of such an item.
The other kind of magic rarely affects the aura significantly.
If you think along those lines cursed items are easy to understand. In fact few are even intentionally cursed, most are just mistakes made by the crafter. For instance the greatly feared gender-swap line of cursed items. Many are actually fine, but most won’t touch one with a ten foot pole, due to the high tendency for the change to be more permanent than intended. In fact, any type of magic dealing with physical change is particularly prone to this problem. Simply, the maker forgot to make it so the effect is reversed when the item is removed.
Rules for aura affecting items. Every item of this type will have a distortion rating. this can be as small as 0.05 or as big as 5-6 for major items. Simply add twice the distortion ratings of all such items active, rounding fractions up, and use the resulting number as a penalty to all spellcasting pools for aura effecting spells(such as healing and invisibilty) as well as to the magic score of magically active beings. After a certain amount of time with the item being active (as a rule of thumb, 1 day per .1 of distortion) reduce the distortion modifier by .05 per week. This continues until the minimum distortion is reached, using the new distortion factor for the interval. This cannot reduce the modifier below the listed distortion. Item tailoring(which usually requires that the item be made specifically for a single person) can also as much as halve the distortion, in which case the minimum distortion is also halved. But anyone else using it takes increased distortion penalties.
Items which have their own auras have no significant interaction with distortion. On one hand this means one can carry a lot of them. But most of these items only have an effect when specifically activated in some way. Simply carrying it does nothing for the owner. Magic armor has to be worn, a magic sword still needs a wielder to swing it, and a wand has to be commanded to fire that magic missile at the darkness.
As a side note, those who use massive amounts of magic items, or high powered magic items, run the risk of permanent changes to their aura. This is very rarely a good thing. Basically imagine your aura has cancer. Or grew a second head. The Negative Qualities they added for cyborgs in the Augmentation book would be a good starting point. Basically I don’t want to force them on players as a general rule, but it could make for a good plot point, especially for a villain.
Note that the amount of loss of actual Aura stacks with the amount of Aura Distortion for purposes of the penalties.