Seer Stones are generally made of a crystal that has been naturally tumbled in rivers, this will give them a frosted look on the outside and a polished "window" to look into the inside of the stone.
Seer stones have been used as an aid to scrying for a long time with the first recorded history coming from the Bible.  They have been used as a divination tool by ancient prophets and it is said they can show you the past, future and present by taking you deep within your inner self or connecting you to the universe.