Oxygen. Bottom line: Skin doesn't "breathe" and has no use for oxygen applied topically.
Collagen. Because it's a large molecule, collagen doesn't penetrate the skin. The only topical products shown to boost collagen levels are retinoids and vitamin C. Collagen is an effective moisturizer—but hyaluronic acid is a better one.
Stem cells. These regenerative cells have incredible potential—but there is so far little evidence supporting their effect when applied on the skin.
Precious metals. Gold has no moisturizing or conditioning effects. Silver has antimicrobial properties that are beneficial for the skin—but so does ethyl alcohol, which costs less.
Snake venom. Most products touting this poison include a compound (branded as Syn-Ake) that claims to keep muscles relaxed. There is very thin data supporting the claim. (See also: bee venom.)
Caviar. Just as fishy as it sounds.