Obsidian is volcanic glass that is highly protective. It is sharper than glass and has been used in the past for blades, arrows and weapons, and is even sometimes used in surgical tool as it is sharper than steel. It is said to shield the wearer against negativity, ground you through your root chakra and can be helped, through meditation, to draw out mental tension and stress. Obsidian is also used for scrying and receiving messages from the other realm.
Each point is 3-4 in.