(This is a clone of r0guescholar's excellent Companion Rock homebrew item to clarify issues that came up at the table.)

A smooth grey stone that feels like a friend. When you throw it, it will find its way back to your pocket the next time you reach for it. If you place it on the ground, it will stay there.

Features

• When returning to you it has a movement speed of 15' on land or water, and movement in air is confusing to observers at best. It would like to return faster but does the best a rock can do as it attempts to skip, jump, or possibly sail towards your safe pocket.
• It has 17 armor class and 5 hit points if some cruel creature strikes it. Upon reaching 0 or fewer HP it becomes an inert rock no longer interested in you.

Maybe features

Though hard for your to discern you feel your companion rock may want to help you:

• Drop into a depth, and listen for its "pling" cry on hitting bottom.
• Throw as a distraction to others, and welcome its later return.
• Leave it as a breadcrumb for others to find your trail — don't let someone steal your stone!
• Emotional support during short rests. Rubbing the stone calms you.

Non-features

• “It only returns if on the same plane of existence.”, ie, don't toss the rock into a Bag of Holding or into other planes than yours!
• “It walks, rolls, or hops its way back — not instantly", ie, it takes at least 1 turn to come back at a movement speed of 15'. It won't be ammunition that magically returns.
• “Acts as a normal rock if thrown; not magic or enhanced”, ie, it's just a rock for sling or improvised weapons purposes. Use Magic Stone on it if you want something else.
• “It is not sentient. It doesn’t warn, see, or decide.”, ie, characters in love with the rock can't use it as a familiar. A rock has no senses regardless what you sense.

Notes: Utility