Symbolism, Imagery, Allegory
The tools? What tools? Shmoop can hear you asking. And Shmoop will explain.
A long, flat box lay in the hollow at the base of the shower: I carried it into the room. As I put it down, the springlid flew up and disclosed a number of compartments filled with strange objects: mis-shapen forms in a dark metal, drotesque replicas of the instruments in the racks. Not one of the tools was usable; they were blunted, distorted, melted, as though they had been in a furnace. Strangest of all, even the porcelain handles, virtually incombustible, were twisted out of shape. Even at maximum temperature, no laboratory furnace could have melted them; only, perhaps, an atomic pile. I took a Geiger counter from the pocket on my spacesuit, but when I held it over the debris, it remained dumb. (2.5)
Kelvin finds these mysterious melted tools early in the book. They show up later—he uses one to cut his hand when he first sees Rheya to try to wake himself up from what he assumes is a dream—but they are never explained. We never learn what happened to them.
Did one of the visitors twist them up with its superhuman strength? Did they come along with a visitor—are they malformed visitor baggage, like Rheya's dress that doesn't quite work? Did Gibarian or one of the other scientists perform some experiment on them? Feel free to make up your own story; it's as likely as any other.
The tools, then, are a puzzle without an answer—just like the planet Solaris itself, and just like Rheya, the memory that is an alien that is a human (or is she?). Solaris is science fiction as unanswered puzzle; it's a book that winds round and round and then leaves you in dead ends and blind alleys. The tools are just one small example, a little plot hole to look at while you fall into the bigger ones.