Cobalt and Magnets

Introducing: Cobalt Ore! A new mineral found below y-level 0 in tuff veins. You must use an Iron or higher tier Pickaxe to obtain the Raw Cobalt from it. You can smelt this ore only with a Blast Furnace and it becomes a Cobalt Ingot, which can be turned into Cobalt Nuggets and vice versa. With it, you can craft Magnets, which can have their polarity switched through Redstone. A North will be attracted to a South, but a North will repel a North. 

You can make a new type of Minecart with Cobalt called a Magnetic Minecart, which has a South at one end and a North at the other. You can put hoppers, chests, furnaces, etc. in them just like regular carts, except that they can be pulled by magnets. 

Maybe you could even make a metal detector with Cobalt, who knows.