Wondrous item, Very rare (Attunement required if 6 or more are worn at a time, to a limit of 20 per attunement slot)
Magically infused gemstones, these rocks seem to compound when multiple of the same type are present. At about 1 inch large, the stones can be either worn socketed into jewelry, or other equipment; or encrusted into weaponry. Worn stones provide a unique buff, that increases with more present. Encrusted into weapons, however, they transform 1 point of the weapon's damage into the associated stone's type, per stone. Changed damage cannot exceed half of damage dealt by an attack. There must be space on the weapon to actually encrust the gemstone in. Trying to put 5 stones on a dagger, for example, is just not practically possible.
The effects of these stones are not immediate. One must acclimate to the stones, which takes 1 hour per stone put on - up until attunement is actually used. When adding stones to pre-existing attunement, one must essentially re-attune the slot to include the extra stones.
Amber. Fire/Homeostatic. Each stone of this type protects the body from extreme environmental conditions. With each stone, the time it takes for you to become affected by extreme conditions increases by 1 round per stone.
With 6 or more stones, one can use an action to extend this time by half a round. With 10 or more stones, one can use an action to extend this time back to its natural maximum. The natural maximum is based only on the number of stones you have, unless another feature has a longer duration.
With 20 stones, one is immune to extreme environmental conditions, similar in effect to the necklace of adaptation.
Amethyst. Force/Flux. Each stone of this type slightly improves one's ability to recover from bad rolls on checks or saving throws. On a roll of 10 or lower, 1 stone adds a +1 to the roll. This improves to 15 or lower for 2 stones, 18 or lower for 3, and 19 or lower for 4. With 5 stones, all rolls for checks/saves gain an extra 1d4 bonus to the roll.
With 6 or more stones, the bonus to rolls is 1d6, and once a minute, gain a charge that may be used as a ki point, spell point (NOT SLOTS), or similar type of energy. With 10 or more, the charges gained each minute increases to 3.
With 20 stones, the bonus to rolls increases to d8, and the charges gained each minute is 9. The number of charges gained are not cumulative. Upon gaining more charges, any previous charges are lost.
Aquamarine. Thunder/Surefooted. Each stone of this type seems to keep one upright. Gain +1 to acrobatics checks per stone.
With 6 or more stones, if an effect would cause you to go prone, you can use your reaction to make a DC 18 acrobatics check to not go prone. With 10 or more stones, this feature is also provided to allies within 5 feet of you.
With 20 stones, as a reaction, if you or an adjacent ally would go prone, you may choose not to instead. The allies use their reaction for this feature, and not yours.
Emerald. Poison/Bodily Health. Each stone of this type slightly improves one's bodily function. Each stone provides a +1 bonus to maximum HP.
With 6 or more stones, the bonus to maximum HP increases to +2 per stone, and you also gain a +3 bonus to Constitution checks and saves. With 10 or more stones, the maximum HP bonus increases to +3 per stone, and the Constitution bonus doubles to +6.
With 20 stones, the maximum HP bonus increases to +5 per stone (a whopping +100!), and the Constitution bonus doubles again to +12.
Garnet. Acid/Bolstered Mind. Each stone of this type seems to improve one's courage. Each stone delays mental-based conditions (like frightened, or charmed) by 1/2 of one round per stone, rounded up.
With 6 or more stones, as a reaction, on a failure to be affected by mind-altering features, you can choose to succeed instead. You can use this feature once per short or long rest. With 10 or more stones, this increases to your Charisma modifier in uses per rest.
With 20 stones, you are immune to the frightened, charmed, and stunned conditions, and cannot have alterations of the mind in any way.
Onyx. Necrotic/Blindsight. Each stone of this type grants blindsight in a range that increases by 1 foot per stone.
With 6 or more stones, you also gain blindsight in a 5 foot square within 30 feet of yourself, that you may choose as a bonus action. With 10 or more stones, you choose 5 of these squares within 60 feet instead. The squares must be of one contiguous area, you cannot choose two spaces that don't connect to each other. the squares that benefit from blindsight are relative to the character, and move as they move.
With 20 stones, the range of blindsight increases to 40 feet, and as a bonus action, this can either be increased to 80 feet or benefit from blindsight in an extra 15(20) squares within 120 feet.
Opal. Radiant/Enlighten. Each stone of this type can cause one to glow 'grossly incandescent'. Each stone causes you and your equipment to glow in a radius of 1 foot bright light and 1 foot dim light per stone, activated/deactivated as a bonus action.
With 6 or more stones, you can use an action to move this light by touch, using an object no larger than 10 feet in any dimension. This source must stay within 30 feet of you. With 10 or more stones, you can instead cause a point within 60 feet of yourself to become 'enlightened', as an action. If this point happens to be an opaque oject, or inside a wall, the light is blocked completely until such an object moves. Your touch-an-object light has the same range.
With 20 stones, the brightness of the light doubles in radius, is considered sunlight, and the range you can use it with increases to 300 feet, as a bonus action instead. As an action, you can cause a burst of light to flash at a creature you see within range. Make a Dexterity check contest against a creature that can see. If the creature fails the contest, it is blinded for 1d4 rounds.
Topaz. Lightning/Slowed Fall. Each stone of this type reduces your rate of descent. Each stone extends falling distance by 5 feet before the falling damage calculation can begin.
With 6 or more stones, falling damage is reduced by 1 per stone. With 10 or more stones, you have resistance to falling damage.
With 20 stones, you are always under the effects of Feather falling.
Turquoise. Psychic/Telepathy. Each stone of this type causes one to be able to broadcast their thoughts. With 1 stone, gain short-range telepathy that affects all creatures who are within 1 foot of your head. This telepathy relies on the fact that a creature knows the language you use to broadcast your thoughts. The range increases to 2.5 feet for 2 stones, and doubles per stone until 5 stones(20 feet at 5).
With 6 or more stones, this range increases by 40 feet per stone(60 feet at 6), as well as the telepathy being able to be to specific creatures. Also, you recognize the presence and number of any thinking creatures (an intelligence, wisdom and charisma score above 6) within range of your telepathy. With 10 or more stones, you can know the exact location of thinking creatures within 60 feet of yourself, as an action. This range increases by 80 feet per stone past 10, and the range of telepathy increases by 120 feet per stone past 9(300 feet at 10).
With 20 stones, the range of telepathy is 1500 feet, and the range of 'prescience' increases to 1000 feet. You are always under the effects of Detect thoughts, though you can't probe into a creature's mind.
Neither feature can pass through more than 30 feet of solid matter or 3 feet of solid rock, and is similarly blocked by 3 inches of metal, or 3/16-inches of lead.
Sapphire. Cold/Breath. Each stone of this type causes one to be able to hold their breath for extended periods of time. With each stone, the time it takes before you start suffocating increases by one minute per stone.
With 6 or more stones, as an action, you can extend the time before suffocation by half a round. With 10 or more stones, as an action, you can instead extend the time to its natural maximum by generating a bubble of fresh air for you to breathe.
With 20 stones, you become immune to suffocation.
Stones of different types compound separately, not together. If you have 10 stones of two types, they do not cause the 20-stone benefit of both types - you would just have the 10-stone benefit of both types.
When giving this item as a reward, consider granting 1d10 of one type. rewarding 20 at once should be considered legendary.