Armour stone is a generic term for oversized boulders or rocks, which are mainly used in the construction of sea walls. An important requirement is of course the harshness of the stone and resistance to erosion.
Granite and Limestone and the most commonly used types of rock, but this also depends on availibility per country. Examples of available armour stone sizes and types include:
LMB 5/40kg LMB 10/60kg LMB 40/200kg LMB 60/300kg HMB 300/1000kg HMB 1000/3000kg HMB 3000/6000kg