You BROKE my Useless Dungeons and Dragons items 

The ring could serve as a good phylactery since nobody, not even the user would know where it went

Imagine if using the ring broke attunement, so you had to find the ring again, re-attune, wait for a long rest, then you could get it to disappear again. Could be an interesting challenge for a ranger by their mentor, like “attune to this ring, use its power, then return the ring to me” requiring them to track down the ring and prove their abilities to their mentor

- suspend infinite pebble over vial of acid with some string or twigs - place vial behind door - hang pouch upside down over metal tray next to your bed - enemy opens door - pebble drops in acid and immediately reappears in bag - pebble drops onto metal tray, alerting you of the intuder Easy, portable intuder alert. You could probably even reuse the vial of acid if you use string to make the mechanism more sophisticated. But even then a vial of acid costs like 10 gold. Could also be used as a phantastic silent alert. - Give pebble to favourit NPC - "If you ever need help, crush the pebble." - "Yo, the pebble returned! The innkeeper must be in danger."

So if you say fired the stone at high enough speed to impact something and get destroyed thus damaging both itself and the entity it could work as infinite ammo. The pebble magic musket will be born.

Now the "deal damage and heal" item allows you to reset death saves lol

I had so much helping you out, for all the good that will do given how wonderfully creative people have been in the comments

I place the pebble under a heavy floor tile in front of the location a portal to the Abyss is known to occasionally open. If anything heavier than a dretch walks out, the additional weight will crush it. Now give the bag to an order of priests. If the stone can be withdrawn, that’s their early warning system. This can be used for a lot of other warning systems, but the general gist is that it’s basically like a tampering seal. Niche, and hard to notice for tamperers - exactly what you want.

The stone of healing would be good for resetting the death saves of knocked down allies.

Bag of infinite pebble can be used as a detection for explosions. Place it down, if it gets exploded it reappears in the pouch. Or detection if a falling boulder trap gets triggered. If the pebble is ground to dust it reappears in the pouch. Quite useful I will say~

With a pebble, unfortunately making it useless is very challenging. The spell catapult from either TCoE or XGtE (I can’t remember which one) uses a pebble

Fantastic video! The ring is BY FAR my favorite item 😂

Suggestion on the damage/heal rock: it doesn't actually damage or heal. It's made of illusion, of shadowstuff. When a creature is struck with it in an attack, it must make a Wisdom saving throw or an Investigation check (I'd allow either to work, like how grappling allows either Athletics or Acrobatics) against a DC of 10, or they become convinced that they just took 1d4 damage. Their belief in this "fact" lasts for a number of rounds equal to the same, and if this would drop them to 0 they fall prone and become dazed until the effect ends. Personally, I might allow minor applications with things like maintaining Rage, breaking Hypnotic Pattern, etc, but I'd force those who know about the rock's properties to have advantage on their check/save whether they want it or not and disallow voluntary fails on the save, and remember they must fail to beat a 10 with whichever modifier is higher (meaning some characters may quickly find themselves only capable of failing if nat 1's are allowed to auto-fail) after being hit (so higher-AC targets are difficult to use this on). Also I'm not entirely certain how I'd have it react to a sleeping character given the way illusions and dreams are intermingled yet the sleeping character wouldn't perceive the rock in the physical world coming at them, so maybe they'd dream of the rock. I'd probably also give advantage to a blind target. Dunno if you saw the responses in my 1st post of the damage/heal rock hence reposting (with some modification), hope you like it!

The healing rock can be used to reset death saving throws back to one if a character has failed multiple. Bag of pebbles is just a free returning item, can be used with the catapult spell for infinite ammo, infinite sling ammo, can be transmuted into various objects, etc. Heels of Action are useful for the simple fact that they are magical boots, which are much more durable than normal boots, and automatically adjust to the user's size, making them just superior shoes to anything mundane. Ring sucks honestly

7:40 I already see 2 good uses for the overhauled rock- You're barbarian, you're raging, but you didn't and can't hit anything, and you didn't take any damage, so, you hit yourself, because you're raging you're resistant to bludgeoning so you heal 1 damage, and then take 1 damage- you end up with the same number of hit points, but you've prolonged your rage. Another usage- the cleric/paladin is down, and they have already failed 2 death saving throws, you reckless attack the cleric/paladin with this rock (to make sure you hit), they immediately get back up with 1 hp, and go back down, but this time with no failed death saving throws.

Make it so the pebble crumbles to dust if moved to fast while outside of the bag, and it's a cloth bag how would it not move along with the weapon and thus still hit the holder, perhaps they take bludgeoning damage instead of other types ...

Ok, the ring doesn't specify that you have to wear it to activate it (Thank Attunement) so the rubber ring can be used as the method of locking a trap in place and then you trigger the trap by teleporting it. Your 8 ounce pebble is still a perfect catapult object Your heal/harm stone is a great death save reset and could be used by a familiar to do so (it drops the stone over the person triggering a Dex save that it automatically fails and it is not an attack. Really you would have been better off using the original format but replace healing with temporary HP this way the best that it could ever be is 3 temp HP. Remember D&D players have Wish as an option so we have to figure out ways of loopholeing everything just to be able to do anything beyond spell duplicating.

Having something like attunement as part of the item, I feel makes it too closely tied to a specific mechanic in 5E. A truly useless item should be truly useless across all systems and genres in my infinitely humble opinion.

Stone + life cleric desiple of life feature is infinite healing

Instead of that stone healing 1 hp make it give 1 temp hp and deal 2 damage.

You seem to adress the problems by adding more layers to them, or changing the basic concepts. That's unecessary. Just keep the rationale as specific and short as possible, and within normal rule logics. And don't be afraid to add limitations from the get go, but make them rule limitations first and foremost, not by adding more effects, as that can always add more problems. The original ring could have easilly been fixed by adding the following sentence: "Cannot be teleported to a space already occupied by a creature or object". The pebble bag could just be a small pouch and most problems would be solved by just that. But you cannot stop people from using it as ammo without writing something like "cannot be used as ammo". Would be too many ad hoc for every possible situation. I would have it keep a pouch full sand, and you can only pull out one grain of sand at any time. But anyway, that fix worked albeit multilayered. But it really is the hardest of your concepts here to work against as it builds on having a bag full of things that actually ARE useful. And anything that can add an infinty of something is always exploitable. For the healing stone, on a hit make it deal 1d4 bludgeoning damage that cannot be resisted or prevented in any way, and then have it heal with the same amount of damage and just add that neither healing nor damage can be modified or changed in any way. Easy. As for the heels, just adding "Once per turn you can...." would have solved it.

Use the ring for a game of 3 card monte. Follow the pebble. 3 cups

Alternatively, allow your players to be creative and think about using seemingly unless items in new ways.

Just use arcane ward on the stone infinite health