Yep, prince rupert* drops are thermally tempered glass. As I understand it (again this is not my field, I just think it's neat), prince rupert drops aren't "frail" at any one point exactly - the long tails are just as strong as the rest, but because they have such a tiny cross sectional area (thus the force you apply isn't distributed across a wide surface area) you're able to overcome the strength of the glass. That's why a failure there causes a cascade, because the force is otherwise evenly distributed throughout the whole surface. Because the glasses don't have a similar feature where the cross section becomes miniscule, they're not going to be significantly more or less frail at any one point.