minetest.register_on_generated(function(minp, maxp)
    local amount = math.random(0, 50)
    for a=0,amount do
        local pos = {
            x = math.random(minp.x, maxp.x),
            y = math.random(minp.y, maxp.y),
            z = math.random(minp.z, maxp.z),
        }
        for i=-1,1 do
            for j=-1,1 do
                for k=-1,1 do
                    if math.random() > 0.2 then
                        continue
                    end
                    local p = {x=pos.x+i, y=pos.y+j, z=pos.z+k}
                    local n = minetest.env:get_node(p)
                    if n.name == "default:stone" then
                        minetest.env:add_node(p, {name="yourmod:stone_with_yourmineral"})
                    end
                end
            end
        end
    end
end)