This magical sack appears to be a bag of holding of one of the 4 quality types. 

It will perform properly for 2-5 uses (or more if the usages are made within a few days time). However, at some point the magic field will waver, and precious metals and gems within the bag will be turned into common metals and stones of no worth. When emptied, the bag will burst to pour forth these transmuted metals and minerals. 

Any magic items (other than artifacts and relics) placed in the bag will become ordinary and dull lead, glass or wood as appropriate (no saving throw) once the transmuting effects have begun.