Absolutely there are different glass recipes. Every mfg has their
own particular recipe. Then you have the Coefficients of Expansion,
not to mention how the beads were made. Were they in layers.
Generally speaking, the higherthe COE, the softer the glass. So a
COE of 104 is going to be much softer than a COE of 33, which is
typically borosilicate glass, such as Pyrex. Because I can’t see the
bead and how it was made, it’s difficult to conjecturewhy it broke
at this point. There is also annealing the bead to consider. It very
well might have been that the bead, when it was made, was not
annealed properly and still retained stress. This is certainly
possible, as beads and paperweights have been known to break or even
explode months after making, because they were not annealed properly
and retained stress within the glass.
I’m not sure how this info will help you in the future, but just
know that it probably wasn’t anything you did improperly, and least
it doesn’t sound like it. One thing you might want to remember is
that you should grind in short bursts, even under water, rather than
persistent drilling. That not only gives the water time to cool the
glass, but gives the burr a chance to get rid of sludge that might
cause breakage. Hope the info helps.