Plattnerite, also known as Wellsite, is a rare and mysterious substance that runs The Time Machine. Without Plattnerite,The Time Machine-- and many other time-traveling vehicles-- would be unable to travel through time. It appeared as a green crystalline mineral, seemingly glowing from within. It can be suffused into other crystalline materials, such as quartz, it can even be in chunky form, like regular crystals if there is enough of it, or it can be filled into flasks, and fuel a machine like blood is circulated through the veins of a human being ("The Time Machine", "The Time Ships", "The Wee Time Traveler")
In a causal loop, Plattnerite was given to The Time Traveler as a young man by an older version of The Time Traveler himself in a medicine jar. Similarly, Andrew McDonald was given his Wellsite from an unknown visitor, presumably a time traveler, later found to be Lara Myers herself as part of a temporal paradox. ("The Time Ships", "The Wee Time Traveler")
However, when he met Lara Myers in the Springtime Era, the Time Traveler recalled having recovered the stuff from a meteorite impact. ("The Wee Time Traveler")
During his experimentation with the stuff, he had his adoptive daughter and assistant Amelia Fitzgibbon order materials with which to construct more of the substance. ("The Space Machine")
Prior to using it for time travel, The Time Traveler had invented a number of simpler machines, including a Flying Machine and a mechanical bicycle, powered by the stuff. ("The Space Machine")
The material is a radioactive substance, and the radiation it emits is part of what helps power the Time Machine, as well as the Warp Bridge. Unfortunately, the radiation also proves fatal to humans with direct exposure over long periods. The substance has a half-life, which accelerates when it's used for time travel. However, it's also self-propagating, and once it's completely decayed it will eventually replenish itself. ("The Wee Time Traveler")