There are two types of diamond deposits: primary and secondary. Primary deposits are those in which the diamonds remain inside the original host rock (usually kimberlite) that conveyed them to the surface. Secondary deposits are formed when the diamonds are eroded from the host rock and concentrated by the action of water into alluvial deposits (in rivers) or marine deposits (in beaches). Stornoway's exploration programs are directed primarily toward the discovery of primary diamond deposits.
In diamond exploration, it is generally accepted that economic primary diamond deposits are associated with ancient shields or "cratons". These cratons are large, coherent expanses of rock that have been geologically stable for at least one billion years. Cratonic "keels" beneath these formations can extend to depths within the earth's mantle where pressure and temperature conditions are favourable to diamond formation and preservation. The structure of these diamondiferous keels is complex which is one reason why kimberlites devoid of diamonds can occur in close proximity to richly diamondiferous ones.