Gemworld. Book 1 - John Vornholt

Lieutenant Melora Pazlar, first seen in the Deep Space Nine episode that bears her name (Melora), is the only Elaysian in Starfleet. Stationed on board the USS Enterprise, six years after her short-lived romance with Dr Bashir, she receives a desperate call for help from her crystalline, multi-species homeworld. When subspace contact fails, Captain Picard sends the Enterprise to investigate, only to find the ship nearly torn apart by a dimensional rift and stranded, badly damaged, below the shields that protect the carefully constructed planet. With both escape and rescue impossible, Picard and crew set out to discover the cause of the rift and the strange, cancerous growth in the crystal choking the planet itself. A Federation ship attempts to reach the Enterprise but is destroyed, and when thoron radiation threatens to kill the crew, all signs on the planet point to sabotage...