This has been a subject of debate for a long time in the online para/meta community.
From YEARS of experience, this is what we've discovered.
Things you can use:
Jewelry polishing clothes
Dish soap and water
Baking soda and water
Things you cannot use:
Harsh chemicals of any kind
Entities do not mind water in the least. A dab of dish soap or baking soda on a wet tooth bush will work wonders.
However, not only can harsh chemicals like bleach, chlorine, ajax, Lysol, and Mr Clean damage the metal and stones of your vessels, the entities do not like it.
Using harsh chemicals will not cause them to leave you permanently, but they will prevent the entity from returning to their vessel until the chemical traces are completely gone!
For some chemicals, this is simply a few days, but for other, more caustic chemicals, this time frame can be months!
Chemicals leave behind a film and this film can prevent your entity from communicating, bonding, and even understanding their human companion. It basically creates a barrier between yourself and them.
If something happens to your vessel, for example, the stone falls out.
If you cannot repair it yourself, don't be afraid to take it to a jeweler.
Make sure you let your companion know that they will be going through the hands of new people in order to repair their vessel. Tell them that its OK that they vacate their vessel until the repair is finished and/or back into your hands.
Following the repair, the most important step is to make sure that they go through a thorough cleansing and recharge so that all of the "foreign" energy signatures are erased from their vessel.
They form a bond with you through your own personal energy and having other energy signatures involved can create confusion.
You can also have your vessel resized if need be, using the same steps as above. BUT, do make sure that a resizing is OK with your entity prior to doing so.
Most entities aid in picking out their vessels after the summoning process. They become attached to their vessels, sometimes extremely so. There are some that will simply not allow it.
Entities are intelligent creatures. They realize that sometimes "shit happens" and that they need to adapt so don't be afraid to communicate with them and get their opinion on what they will and will not allow concerning vessels.
Remember, the vessel belongs to you AND them so decisions must be made together.