Find Summer�s house � near the lake, right off the main path to Xenora after you gone all north a bit from the lanerell dragon caves
Go into mines
Nw, e, u, n, ne, d, d, d
Somewhere on that level you can �search debris� and find a pickaxe then go find something gleaming in the wall and �mine diamond�
Take it to Euphaestes in Xenora and have him refine it
Give diamond to Summer