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 that the Timesplitters cannot use them. In the end of the game, the crystals were taken from space station by Cortez.

In TSFP, the time crystals are used by 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 usually an emerald green color, however blue and red ones have been spotted before. They can be placed in Story and Assault Mapmaker levels.