Ryan and ScarJo -- Lowballin' the Bling

We know this is going to sound ridiculous, but Scarlett Johansson's engagement ring is actually blingier and more ballin' than she's letting on.

The buzz on ScarJo's bling is that the ring -- designed by Sonia Boyajian -- is around three carats and cost about $30K. Not so, we're told -- the ring is actually "north" of four carats -- and set Ryan back a cool $120,000! Get a piece of that rock.