Spears were thrown at the start of a battle. The Aztec people built up a large empire in what is now Mexico between 1400 and 1510. Their warriors won a series of battles in these years against other American peoples. Each battle began with men on both sides throwing light javelin at the enemy. Then the men would charge at each other to fight at close quarters.
Obsidian stone was razor sharp. The Aztec, Maya and other peoples of America did not know how to make iron or bronze, so they made their weapons out of wood, stone and other natural materials. The most effective weapons were edged with slivers of obsidian, a very hard stone that has a very sharp edge when first broken.
Spears were tipped with stone. Spears were often tipped with flint, which was easier to shape and found in greater quantities than obsidian. These spears were designed to wound the enemy so that he could not fight so well.
from "100 Things You SHould Know About Arms and Armour" by Rupert Matthews