Ring of Acuity is an enchanted ring which adds two level 2 spell slots, one level 3 spell slot and one level 4 wizard spell slot. This item appears in Baldur's Gate II: Shadows of Amn and this item will probably appear in Baldur's Gate II: Enhanced Edition. It's dropped by Lavok when the party fulfills his wish to be on the normal plane once again.
The origins of this ring are unclear, and while its enchantments share similarities with historical examples of Rings of Wizardry, there is something strange in how it feels, either in the weight of the metal or in the aura it projects, and it does not function the same. It was likely found on some distant plane, and as such, its maker will remain a mystery to you.