Chert is a type of rock that includes flint and agate. It sometimes has fossils in it, usually shells. It is a sedimentary rock, and it is very hard. It is made from quartz. People used chert to make arrow heads because it was hard and sharp when broken. The color of chertz is usually white or gray, but can be red, yellow or dark grey. If there is a lot of iron oxide in the chert it can be dark red; this type of chert is called "jasper."