“Heart of Stone” is episode fourteen of season three of Star Trek: Deep Space Nine.
Major Kira and Odo are returning to DS9 from a Bajoran colony on a runabout when they receive a distress call from a ship that has been attacked by a Maquis vessel. They chase the Maquis, but the vessel essentially crashes on a planet. When Kira and Odo set off in pursuit of its pilot – separately, of course, in violation of common sense – Kira gets trapped by some sort of crystal.
The crystal is growing, and will eventually cover her completely. Odo tries to find a way to remove it from her, but all his efforts fail. When Kira is completely covered, she will die.
Back on the station, Nog has approached Commander Sisko. In Ferengi terms, he is now an adult, and is expected to buy an apprenticeship. Nog wants to join Starfleet, even though he is a Ferengi and not a Federation citizen. Jake thinks Nog is joking, but he is actually quite serious. Of course, you don’t need to buy entrance to Starfleet.
There’s a reference to Nog’s father, Rom, being a mechanical genius. Back in “Babel”, Odo stated that Rom couldn’t fix a straw if it was bent. That has changed.