Catastrophea by Terrance Dicks

The real name of the planet is Kastopheria, but it has been generally rechristened Catastrophea and it is a catastrophe waiting to happen. The supposedly civilized races are exploiting this world, fighting over its wealth and resources, while the indigenous population, golden-skinned giants, don't seem to care what their fate might be. The Doctor and Jo soon become embroiled in a plan to keep peace between different parties vying for control over the planet.

But what is the strange glowing crystal lying hidden in impenetrable jungle? How will the arrival of the proud, warlike Draconians affect the fragile peace? In his quest to find the truth behind the secret history of the People, the Doctor risks unleashing a force more terrible than the galaxy has known for eons...