Grimstone can only be mined with iron pickaxes or higher

I think it would be cool if you needed a iron pickaxe or a higher tier to mine the grimstone similar to only being able to mine diamonds with an iron pickaxe or higher. Since if the grimstone is related to the deep dark you need higher tier gears like iron than just using some wooden tools that cannot help you defend from the deep dark. Grimstone only being mined with an iron pick or higher can be like a recommendation to the player not to go down until they are wearing iron armour and using iron tools.