Cosmetic pets (and DH's Ferret Companion rune) collects gold for you within a limited radius.

Other pets (including DH's Ferret Companion rune) based on your class' skill (e.g. WD's Zombie Dogs, Crusader's Phalanx, etc.) will attack while in combat.

EDIT: I believe the Legendary Gem you're referring to is the Enforcer, which boost pet damage by X%, and its level 25 effect reducing incoming damage by 90%.

To know if a pet is a combat-ready one, it should be visible at the top of the screen, same row with your player icon. Certain pets summoned by Legendary effects such as the Two-Handed Sword, Maximus will benefit from this gem.

This post has been edited by Kevmeister: Dec 4 2016, 10:03 PM