One of the most annoying things that can happen is to have something you're wearing or carrying snag on your clothing or hair. It's minor, in the scheme of things, but it's an uncomfortable feeling and can damage delicate fabrics. A ring that's suddenly started to snag items is a ring in need of repair. If the ring has a stone held in the setting by prongs, chances are the prongs have been bent out of shape slightly or are starting to wear down.
Most Likely, It's the Prongs
When rings are produced properly, they shouldn't snag on clothing or hair. Of course, if you have a high-profile ring with a design or stone that sticks up from your finger, the ring my catch on clothing if you do something like reach your hand into a tight pocket. But those rings still shouldn't catch on fabric if you merely brush the ring against it.
The prongs can wear down; even though the metal seems tough to you, in ring form, it's not a super-strong material, especially if the metal in question is gold. When the tips deteriorate, they can be easier to accidentally bend — and even a microscopic bend can be enough to let the prong start snagging on threads. (This also means that the stone can start to come loose, so it's a sign the ring needs to be repaired quickly.)
Repositioning and Retipping
If all that happened is that the prong was bent back a bit, and it's not really worn down that much, the jeweler might just be able to reposition and file down the prongs. However, if there is noticeable wear, the jeweler can do something called retipping, in which a new tip is created for each worn prong. These are attached permanently and then filed so that the prongs won't snag anymore, and the stone remains secure.
Metal Surface Damage
Occasionally, the cause of snagging is metal damage, such as metal being yanked out of shape and acquiring a sharp corner, or something impacting the metal and creating a sharp dent. Damage in which the metal is merely bent may require just some reshaping. However, damage that results in scraping or cracking may require actually creating a new setting for the ring if the metal can't be repaired smoothly. Much of the time it can be filed down, for example, but it really depends on the type and severity of the damage.
If you've noticed your ring starting to snag items, bring it to a jeweler's and have them look at the ring. The jewelers there should be able to repair the ring in a reasonable amount of time. For more information about diamond ring repair, speak to a jeweler as well.