It seems that the early statues are small and made of more local stones (limestone, steatite and redstone); later, when wide-ranging trade-connections had been established, the more costly exotic diorite was used. Unlike the local stone, diorite is extremely hard, and so difficult to carve.
Initially, sculptures were modest in size and made of readily available stones like limestone, steatite, and redstone. As trade networks grew extensive, they began using more expensive materials such as diorite, which is a harder stone that proved challenging to work with.