Needed for achievement.
The Soapstone is a flat star shaped item found at the very beginning of the Mountain of Madness when you enter the book, right before going down through the door to begin your journey. The description for it says "A green, star-shaped fragment of Soapstone. It has no effect." Just like it says, it has no effect. It offers no special ending, like the sacred items (Holy Symbol, Consecration Kit, and Sandles) in the other worlds. Having this equipped when entering the plane will not alter the ending. It could be that it's just a simple reference to H.P. Lovecraft's books. In an excerpt from the book, at the Mountains of Madness, it describes the Soapstone with detail.
"Have found peculiar soapstone fragment about six inches across and an inch and a half thick, wholly unlike any visible local formation. Greenish, but no evidences to place its period. Has curious smoothness and regularity. Shaped like five-pointed star with tips broken off, and signs of other cleavage at inward angles and in centre of surface. Small, smooth depression in centre of unbroken surface. Arouses much curiosity as to source and weathering. Probably some freak of water action.Carroll, with magnifier, thinks he can make out additional markings of geologic significance. Groups of tiny dots in regular patterns. Dogs growing uneasy as we work and seem to hate this soapstone. Must see if it has any peculiar order." Throughout the story, the character, geologist William Dryer, continues to dig up and find more fragments of soapstone time and time again until he comes across a whole big pile of them. Seeing the imitation of Soapstone dots on an array of other things (like grave mounds) and finding a bizarre objects for decoration carved entirely from this material, he later believes that Soapstone had to do with something significant, but also theorizes that it was a form of currency between different cities.
The Soapstone can alternately be called the Elder Sign, and was mentioned in other works such as Lovecraft's "The Shadow Over Innsmouth" by an old drunk (In this story, the Deep Ones cannot harm a person who wields one), August Derleth's "The Lurker at the Treshold," Dungeon's and Dragon's, Chaosium's Call of Cthulhurole-playing game, video game Call of Cthulhu: Dark Corners of the Earth, boardgame Arkham Horror, and in Lin Carter's short story "The Horror in the Gallery," and in the Book of Iod (apart of the Cthulhu Mythos collection) is described as a powerful weapon used against the servants of Cthulhu and the Outer Gods to drive them off.
After acquiring the Soapstone and returning to the Library, should the explorer talk to the Star-Spawn shopkeeper in the third book they will receive not only a friendly greeting but also a hidden achievement.