If the object of your affection cares more about chemistry and physics than chocolate and flowers, for $75 you can skip the card and get them this tiny aerogel heart that weighs next to nothing.
Developed as a remarkably efficient insulator, aerogel is 96 per cent air by volume, so it's impossibly lightweight. However, it's also difficult to manufacture, which is why this tiny 1.4-inch heart is so expensive. But it's as unique a valentine as you're ever going to find, and what better way to say I love you than with a gift that comes with a six-page safety and hazards identification data sheet? [BuyAerogel.com via Wired]