The Arcane Focus is a magically charged emerald that drops in a Spellweaver's backpack as a result of the successful casting of the Arcane Circle spell. The crystal is a No-Drop/No-Trade and timed item.
The Arcane Focus is a virtual physical representation of the Arcane Circle buff/de-buff; as long as the crystal is in an Arcanist's backpack, the same benefits of the spell are in effect. When the timer expires, or if you remove the crystal yourself, the effect ends.
Holding an Arcane Focus increases duration, spell damage, healing spell effectiveness, and area of effect for the carrier's Spellweaving spells.