I am so going to make this somebody's familiar.

It's a very FAMILIAR rock, am i right?

I'll see myself out.

Oh, do I see endless magical sling ammo?

This is just so cute

I think my companion rock will have cubic form and a heart on it :)

nailed it 

his name is dwaine

If you place it on the ground, it will stay there

I don’t think that’s a magical property…

Oh it is, nothing can move the rock.

Can gravity

Sadly, I found myself making a homebrew clone. This is such an awesome item!

At the table, questions arose from shenanigans:

• “It only returns if on the same plane of existence.”, ie, don't toss the rock into a Bag of Holding or other planes!
• “It walks, rolls, or hops its way back — not instant", ie, it took at least 1 turn to come back. It won't be magically returning ammunition.
• “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. :wink:

I've made an updated version to address some shenanigans.

https://www.dndbeyond.com/magic-items/10312354-companion-rock

Not sure the link works in comments so I kept the item name the same, "Companion Rock".