I take the sapphire crystal and upgrade to a CATALYST, not tear! Catalyst gives you over 200 health and mana every 2 minutes or so which is HUGE, and it also increases your health significantly making it a lot harder to kill you.
Also, my philosophy with Ryze is to be a Tanky DPS that tunnels squishies. A fast Abyssal can make or break that because Spell Flux makes stacking flat reductions that much more impressive. My core usually extends like this: Abyssal>Hextech Revolver>Archangels>Will of the Ancients