In Ch. 9, we see that Karana begins to make weapons because Ramo has been killed and she needs to be able to defend herself from the wild dogs on the island. She gathers enough wood on the beach to be able to construct a bow and arrows, along with a small spear. The spear is missing an arrowhead at the tip, though, because she is unable to find one. She simply sharpens the tip of the wood to a point.
Later, in Ch. 12, she makes an even stronger bow and arrows. She also makes a stronger spear, but she needs the tooth of a sea elephant for the tip. Normally, the men acquire these by trapping the sea elephants in a net, so her trying get one on her own is daunting. She injures her leg trying to do so, and it is not until Ch. 14 that she finds a dead sea elephant on shore and manages to get a tooth out of it for the tip of her spear.
So, her weapons are made of wood, bones, and teeth that she manages to cull from the island.