Obsidian is an item that can be collected from your world. (drag it into your inventory like big rocks) It can be used to make arrowheads, which in turn create arrows to shoot from your bow.
Flint Rocks are throughout the world next to rock piles. (these piles are found everywhere! Dont need to look far.)
Players can use a Flint Rock to craft an arrow head and a bow drill.
Lifting the Flint Rock can reveal a Worm.