Source Core Rulebook pg. 628 4.0
When you Invest an Item that has the apex trait, it improves one of your ability scores, either increasing it by 2 or to a total of 18, whichever grants the higher score. This gives you all the benefits of the new ability score until the investiture ends. An apex item grants this benefit only the first time it’s invested within a 24-hour period, and you can benefit from only one apex item at a time. If you attempt to invest an apex item when you already have one invested, you don’t gain the ability score increase, though you do gain any other effects of Investing the Item.


Amulet of the Third Eye, Anklets of Alacrity, Archivist's Gaze, Armbands of the Gorgon, Artificer Spectacles, Avalanche Boots, Bangles of Crowns, Beguiling Crown, Belt of Giant Strength, Belt of Regeneration, Bracers of Hammers, Breastplate of the Mountain, Circlet of Persuasion, Coronet of Stars, Cowl of Keys, Diadem of Intellect, Dragon Handwraps, Emberheart, Genius Diadem, Headband of Inspired Wisdom, Laurel of the Empath, Mantle of the Amazing Health, Mask of Allure, Mercurial Mantle, Pilferer's Gloves, Purloining Cloak, Sage's Lash, Sandals of the Stag, Sash of Books, Scapular of Shields, Shadowmist Cape, Titan's Grasp, Troubadour's Cap, Viper Rapier, Whispering Staff