Ouch my teeth!

I like to chew on seeds sometimes while I'm pondering what to do next. So this afternoon I reached in my backpack for some watermelon seeds and popped them into my mouth, not really paying too much attention to what I'm doing, untill... Ouch! I bite on something very hard. Turns out I'm chewing some onyx bolttips! Why oh why do they have to look so much alike?