Mercury stone is one of the God Stones Lara gets in Tomb Raider Chronicles from Larson and Pierre in the beginning cinematic of the Rome Section. Or actually it's the centre piece of a stone of the same name that's already placed in it's receptacle in the beginning of the adventure.
Lara, Larson and Pierre meet in an opera house, Lara is going to exhange money for the stone. However, Larson and Pierre try to fool her but Lara manages to take the stone from them and escape with it.
The Mercury stone is needed with the other God Stones to open the Philosopher's Gate.
Despite its similar looks especially in FMV sequences, the Mercury stone shouldn't be mistaken as the main artefact of the Rome section, the Philosopher's Stone.