I’ve been coming across videos of people breaking obsidian chunks to make arrowheads but could never find the answer why it could be broken with light hits with a rock, and then not break when in use with said weapon.
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Obsidian can be made very, very sharp, by breaking it along particular lines. They can even be made sharper than metal blades, and hold that edge better, and surgeons used to prefer obsidian scalpels for that reason. But they’re very hard to sterilize, which is why you don’t typically see them in medical settings anymore.
This sharpness, though, has made obsidian very desirable for use in cutting tools since ancient times. Arrowheads were especially popular, because the arrowhead isn’t expected to last for more than one use anyway: fragility isn’t a problem if the arrow can break through the target’s skin once, and arrowheads can manage that. Fighting knives are more iffy, because the high forces involved in fighting can break a blade easily, but they did see use in small utility knives that didn’t need to stand up to those kinds of forces. Obsidian razors for shaving have also been historically popular. Mesoamerican civilizations had the *macahuitl*, a kind of cross between a club and a sword with slots to hold replaceable obsidian blades. This helped mitigate obaidian’s fragility by slotting it into a more durable backing: if a blade breaks, you just put another one in, and even if you can’t do that right away, the weapon still mostly works.
But it basically all comes down to obsidian’s unmatched sharpness. Even today we haven’t made anything with a sharper edge. Dust from working with obsidian is actually quite dangerous, because the fragments can cut your skin in ways most kinds of rock dust just *don’t*. And people want to use tools with that kind of edge. Obsidian’s fragility is a problem, just as you say, but for some tools, it’s worth putting up with that.
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