Mystic gem capable of fast casting True Seeing.
Acquisition[]
It's one of the new items in 
Baldur's Gate II: Enhanced Edition – Shadows of Amn (2013)
This icon indicates content from the Shadows of Amn campaign of the Baldur's Gate II: Enhanced Edition., obtained during Rasaad's Journey sidequest as a reward for passing the Blinding sun trial in Heretic Temple.
If you've missed the Gem of Seeing in Shadows of Amn you can obtain it again at the beginning of Throne of Bhaal. When you are taken to the Pocket Plane use the Fate Spirit to summon Rasaad and ask him to join you as your companion. He will have the Gem of Seeing equiped in one of his quick items slots. (If you already have the gem you can get a second copy of it)
Description[]
| “ | This finely cut and polished stone is indistinguishable from an ordinary jewel in appearance, although a Detect Magic will reveal its enchantment. When gazed through, the gem of seeing enables the user to detect all hidden, illusionary, invisible, astral, ethereal, or out-of-phase things within viewing range, as though <PRO_HESHE> were affected by a True Seeing spell. | ” |