[Coal. I assume the Londonderry family wealth came (mainly?) from coal mined in the north-east of England, near Seaham (their own creation).
When he was about 13 [check date in Whitehead above], young EVT had a new pit named after him at old Durham - the Ernest pit was in sight of Durham Cathedral. How long was the name used for?]
There is a considerable amount of information on the creation and demise of the "Vane Tempest" pits from these links: