The Clock

 is a key to opening the secret locations of the hidden halves of the Triangle of Light. It also contains a tiny speckle inside of itself, needed for the assembly of the two halves. Lastly it serves as a countdown clock for the planetary alignment.

The clock was found by Richard Croft, who hid it inside a regular clock, from the Illuminati group he was a member of. And then packed it into a hidden compartment in his mansion for Lara to find.