[quote=@The book of bad juju]
It could be kind of rage-inducing, given that for most if not all magical girls one tap on the gem and they're dead. Nobody likes being taken out with just one attack.
[/quote]

Sayaka's gem survived a fall off a bridge without breaking, so I think we can assume that they're not quite baubles in terms of fragility (depending on what was in the truck, of course). The ones which have broken over the course of the series have (as I recall) been either chewed up and broken by witches or targeted specifically by other magical girls at a very close range. So their palm-sized nature does help a lot in that case.

I put Kei's on an anklet specifically to match up with the Achilles' Heel metaphor. :dreamy