Dense fossil is a representative of stackable currency class. If you insert the item into the resonator, it can add more energy shield, armor and evasion mods. However, it has nothing to do with life modifiers. In Path of Exile game the fossil increases the weights of mods with life tags by 0 and defences tags by 10. You can buy a POE Dense fossil either for 1, 000 Azurite from Niko or obtain from Fungal Cavern or Petrified Forest. There is also an opportunity to exchange it for an Orb of Alchemy.