Arcanite is a magical crystal, taking on many different forms and having just as many different effects. It is crystallized aether, a gem literally formed out of solid magic. It appears seemingly at random in hollows or crevices in aether-rich areas, and sometimes forms as a crystalline “pearl” inside especially magically-potent creatures.
Pieces of Arcanite are effectively equipable superpowers. Each grants you one or more passive or active abilities so long as you are Attuned to them. Unlike Magic Items, you can’t craft Arcanite out of base components — you have to find it out in the wilderness, either inside of monsters or in a particularly magical location. Once you have a piece, a Gemcutter can use Reshape Arcanite to modify its powers slightly.
You can be Attuned to a maximum of 2 pieces of Arcanite at a time.
You can end an Attunement to a piece of Arcanite during a Short or Long Rest.
You can Attune to a new piece of Arcanite (including the process of ending your Attunement to one of your current Arcanite) over the course of a Long Rest.
Only one person can be Attuned to a given piece of Arcanite at a time.
You must have a piece of Arcanite on your person — usually worn as a piece of jewelry or embedded in a suit of armor or weapon — and not inside of an extradimensional space in order to use its active Powers and gain its passive benefits.
Spirit Companions can gain the benefit of Attuning to Arcanite, but any pieces of Arcanite they Attune to count against their bonded partner’s limit of Attuned Arcanite — if your Spirit Companion is Attuned to 1 piece of Arcanite, for example, you can only Attune to 1 piece yourself.
Any time a piece of Arcanite allows you to cast a spell that involves an attack and/or damage roll, you make an Arcanite Attack roll rather than a Spell Attack roll, and the spell effect counts as dealing Arcanite Damage of the specified element, allowing you to add your highest Attribute Modifier to the damage roll.
Each property of an Arcanite grants either a Lesser, Greater, or Passive Power.
The Tier of a piece of Arcanite determines how powerful it and the properties that it grants to its bearer are, as well as how valuable it will likely be on the open market.
Tier 0 (Fragments) - x1 total Tier’s worth of properties, 100gp
Tier 1 (Shards) - x2, 1,000gp