Fountain
Fountains are locations where the crystals flow up to the earth's surface and this is the place to harvest them.
On the Map you see where fountains are.
You harvest crystals by placing Workers on them, by clicking and select place worker. But you do need Workers, of course.
Size?
The fountains contains a an amount of crystal with a maximum (varying between 20 and 34 on the types)
The image of the fountains vary in size according to how much they contain right now.
Restore time
When fountains are empty they disappear from the map, but don't worry! They will come back after 24 hours, or faster if several players have used them in the last time period.
Also if a half-empty fountain has been unused for 24 hours it is restored to it full state.