A Zen Stone is named after Zen, a chronurgy wizard who possesses great knowledge and even prophecies of events to be aware of. Many types of Zen Stone exist, each type a distinct combination of shape and color, and usually created by Zen for a very specific character to hold for their lifetime in order to influence the timeline in a subtle way. Though there are so many of them that it isn't unheard of for an unintended person to have possession of one.

ZEN'S STONES: All of Zen's stones have the ability to be used to create a chronal shift with one charge that recharges after a long rest, and this does not require attunement, you merely have to have the stone in your posession. To use this ability you use a reaction and speak the words Chronos Maniputale and you can magically exert limited control over the flow of time around a creature. After you or a creature you can see within 30 feet of you makes an attack roll, an ability check, or a saving throw, you can force the creature to reroll. You make this decision after you see whether the roll succeeds or fails. The target must use the result of the second roll.

ATTUNEMENT: Once attuned to this item you can use an action to toss it in the air and it will hover 3 ft above your head granting you a special gift as long as it does so.

A stone has AC 24, 10 hit points, and resistance to all damage. It is considered to be an object that is being worn while it orbits your head.  If it ever has 0 hit points it falls to the ground and appears to be a normal stone.  It is no longer attuned to you.  It will regain it's hit points and all it's power when you take a long rest and you may attune to it again during that long rest. 

Absorption. While this pale lavender ellipsoid orbits your head, you can use your reaction to cancel a spell of a level equal to or lower than 1+your spellcasting modifier cast by a creature you can see and targeting only you.  That spell is then stored in the stone and you may choose to cast it without using any components, but you do expend the spell slots necessary to cast it.  If you do not have the available spell slots you may not cast the spell.

Once the stone has canceled or cast 12 levels of spells, it burns out and turns dull gray, losing its magic. If you are targeted by a spell whose level is higher than the number of spell levels the stone has left, the stone can't cancel it.  The stone can regain it's power during a long rest but in order to do so you take a point of exhaustion and you do not regain any used spell slots during that rest.

Notes: Warding, Consumable