How would I generate stone underneath my grass terrain?

Hi, i'm trying to make a terrain generation generating stone. I've made the grass part but no idea how to make stone appear. If you can help, I'd really be thankful.

local seed = 7345384535677213

for x = 1,150 do
    for z = 1,150 do
        local object = Instance.new("Part",workspace)
        object.BrickColor = BrickColor.Green()
        object.Anchored = true
        object.Size = Vector3.new(3,3,3)
        object.Name = "Grass"
        object.Material = Enum.Material.Grass
        local y = math.noise(x/20,z/20,seed) * 5
                object.CFrame = CFrame.new(x,y,z)

end
wait(.05)
end