Appearance enhancers are amongst the few magical items that really are always jewellery of one form or another, although this appears to be more of a cultural tradition than a magical requirement. Most are enchanted rings, although broaches, studs, bracelets, anklets, lockets, and a variety of other items are not unknown, and all of these seem to work equally as well.
The magics required to make an appearance enhancer are fairly easy to enchant (any first year magic student can do it), so they are one of the more common magical items. They are also relatively inexpensive (as magical items go), typically only 10-20% higher than the price of the base item, making them available to all but the poorest members of society.
Their easy availability and relatively low price have made them into something of a "must have" item in most civilised cultures, all but replacing the cosmetics industry while providing a steady income for traders in magical trinkets the world over.
Appearance enhancers allow the user to cast a range of cantrips, glamours, and petty illusions designed to produce minor alterations in his/her appearance. Effectively allowing the user to wear a face that accords with current fashions, although repeated castings may be required to continuously maintain the effect.
Note that the user could also cast them on other people, possibly to make them appear less attractive. In any such cases normal resistance rolls (as dictated by the rules of your preferred magic system) should apply.
The spells involved, being not particularly powerful, can effect only cosmetic changes (colour of hair, eyes, or skin, length or colour of eyelashes, colour or fullness of lips, etc), mask minor imperfections (such as unsightly spots or birthmarks), or add attractive markings (such as beauty spots). All effects are however relatively short lived, typically lasting no more than an hour or so (and often much less).
The spells do not (and indeed cannot) in any way alter the basic shape and structure of their target, thus the target always remains recognisable for who and what he/she truly is.
Furthermore the spells work only on living creatures, any attempt to use them on non-living matter (such as clothes or armour) automatically fails. Thus the targets’ clothes remain unaffected (if you want your clothes to look good, you’ll just have to wear good-looking clothes). Note that this means appearance enhancers cannot be used to appear unarmed while sneaking into the king’s bedroom carrying a heavy crossbow, so don’t try it.
Note: Some "always on" versions exist that maintain the illusions indefinitely (or until the user specifies a change). However these require a piece of Crystallised Dragon Blood to power the effects, making them prohibitively expensive and available only to the very richest people.