Except in special circumstances as in material requirements for a spell, a focus can be used a number of times according to the following formula:
(Size/useage)x(quality)
So a perfect small gem used to its maximum will have less potential uses than a flawed bigger gem used lightly.
Example, a perfect gem used to its maximum extent so its capable of 3MP and has 3MP stored in it when used will have to pass a 1d100 roll DC2. This means only a natural 1 will destroy it.
