Chert

Chert is a type of rock that includes flint, agate and quartz. It sometimes has fossils in it, usually shells.

It is a sedimentary rock, and it is very hard. Sedimentary rock is formed when sediments are compacted and cemented over a long period of time.

Historically, chert was used to make arrow heads because it forms hard and sharp edges when broken. 

The color of chert 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."

References

Category:Sedimentary rocks