Mine in Iron Lake, the majority of the mining points are close to the Crystal, and far from aggressive mobs. Cobalt ores are easy to mine. Sweet Spot is -4, and you have to hit the last 1/5 of the circle. You will also get lot of Iron which you need to make the Cobalt ingots, use the extra iron for iron nuggets which you'll need 40+. (Iron ore sweet spot is still -3, hit ~3/4)

That's what I did 30+ and found it way better and cheaper than doing Iron Celata Visors.