From The TimeSplitters Wiki

Time crystals are important items in the Timesplitters series of games. Time Crystals are used to travel through time and were used by timesplitters in TS2 in their quest to destroy humanity. Cortez must collect the time crystals so the Timesplitters cannot use them. In the end, the crystals were destroyed alon with the space station.

In TSFP, the time crystals are used by Cortez in order to go back in time and prevent the timesplitters from ever being created. Crow also uses them in his time device while searching for immortality. He drilled for the crystals on the same island Cortez goes to in Scotland The Brave.

Time crystals are an emerald green color and can be placed in Story and Assault mapmaker levels.