redstone does not exist in Minetest; that is a Minecraft item. Diamonds are generally only found very deep underground, past a couple hundred nodes deep orso. The most sure way to find ores is to dig several long parallel tunnels one block wide and several blocks tall, which have stone no more than two blocks thick between them - that way, all ore blocks along and between the tunnels are exposed. Like this:

So long as you're several hundred blocks underground, using this method you are sure to find several diamonds within the first few minutes of digging - but it can become tedious after a while, and you will never need to go mine for cobblestone again because your storage will be always overfilling with it.