OPINIONS: The Gemara cites the Mishnah in Mikva'os (4:1) that teaches that certain types of vessels invalidate rain water from being used for a Mikvah by giving it the disqualification of drawn water ("Mayim She'uvim"). Among the vessels listed there are Klei Gelalim. What are "Klei Gelalim"?
(a) RASHI explains that this is "Shayish," which normally means "marble" in Hebrew. We find this word in the verse, "Even Glal" (Ezra 5:8). (TOSFOS in Bava Kama (2b) cites RABEINU CHANANEL who interprets the word "Galal" in a different verse (Ezra 6:4) to mean marble. However, this does not mean that he also translates the word "Gelalim" in the Mishnah in Mikva'os to mean marble as well.)
(b) However, all of the other Rishonim, including RASHI himself elsewhere (such as later in Shabbos on 58a) and TOSFOS (Menachos 69a), explain that Klei Gelalim refers to vessels made from dry animal dung. The Rishonim cite three proofs for this interpretation:
1. The RAMBAM (Perush ha'Mishnayos to Kelim 10:1) says that if Gelalim means marble, then it is already mentioned in the Mishnah, because the Mishnah lists "stone vessels," and marble is a type of stone.
2. TOSFOS in Menachos (69a) points out that the Gemara teaches that if an elephant swallows a basket and eliminates it in its waste, the basket has the status of Klei Gelalim. If Klei Gelalim are marble vessel, then why should the basket that comes out in the waste of an elephant be considered Klei Gelalim?
3. The RASH MI'SHANTZ to Uktzin (2:10) writes that the Mishnah there says that if vessels made of "Adamah" (earth) or Gelalim are soft enough to allow roots to penetrate through them, they are considered to be utensils with holes in them. The Mishnah specifically omits stone utensils, because stone utensils are never soft enough to allow the roots of a plant to penetrate. If Gelalim refers to marble, then the Mishnah should also omit Gelalim, because marble is even harder than ordinary stone! It must be that Gelalim refers to utensils made of dried animal dung.
The NETZIV in MEROMEI SADEH suggests that even Rashi here understands that Klei Gelalim are vessels made from animal dung. The word "Shayish" that Rashi quotes is a foreign word, and not a Hebrew one, that means animal dung. (Perhaps the Netziv is referring to the German word for dung.) Accordingly, all of the Rishonim agree that Klei Gelalim are vessels made from dung. (However, when RABEINU ELYAKIM in Yoma (2a) explains what Klei Gelalim are, he cites two different explanations and says that they are either vessels made from "Shayish" or vessels made from dung.)