They took the form of an Egyptian sarcophagus and were entrances into a time-space tunnel. When activated, the coffin shaped device lost its stone form and became a visible doorway with energy spiralling in its form.
Lodestones differed from most forms of time travel in that there was a 1:1 ratio between the distance in time travelled and the time experienced for the traveller; for example, a person on a journey 70 years back or forward in time through a lodestone would age 70 years. As the Osirans lived thousands of years, this was not a concern to them. (PROSE: The Sands of Time)
On Donna's World, Rose Tyler and UNIT created a time machine called a Lodestone using the surface technology of the Tenth Doctor's TARDIS to send Donna Noble back in time so her younger self could turn left at Little Sutton Street. They did so by bouncing chronon energy off of set of mirrors back into the centre, in which Rose and UNIT controlled and decided the destination. Donna did not age visibly during this trip, nor did she appear to take two years from her point of view to travel two years back in time. (TV: Turn Left)