I could tell you where it is, but that would spoil all the fun...
This page contains spoilers! Read on your own peril.
The Heart, also known as the The Burrick's Heart Ruby, is a huge gemstone; the largest ruby ever found. Although usually deep red, it may appear lapis in certain light. One of the older sentient artifacts, it possesses enough sentience to talk, via telepathy, though it is rarely heard.
History[]
Nothing is known about the origin of the Heart but it is known that at some point in time lord Clive Warwick brought it back with him from his last excursion after which he wasted away, entranced by its powers.[1][2] It was subsequently donated by his family to the Wieldstrom Museum and passed on as an ordinary museum exhibit where it bides its time by entrapping and polluting the minds of susceptible humans.[3]
Deadly Shadows[]
Garrett is required to steal the Heart from the Wieldstrom Museum (along with The Eye and possibly the the Crown) and use at the end of Thief: Deadly Shadows to activate The Final Glyph, by placing the Sentients (one whom is the Heart) in certain landmarks of The City, and destroy all glyph Magic. Its receptacle is the Bradshaw Monument in Auldale.[4]
Garrett can also purchase a replica of the Heart at Carmen's Place prior to the museum heist so that the museum guards don't notice the absence of the artifact.[5]