Please see http://www.yeshiva.co/ask/?id=7293 On Shemos 16:14, Rashi says gir is black. How does this tally with Yeshaya referring to limescale. Is there such a thing as black limescale?
ב"ה Shalom Your question is well in place and you have indeed stumbled upon a Rashi which needs explanation and you have caused me to do some "detective" work. First of all the word "gir vezarnich" which Rashi in Shemot refers to as black does not appear where Rashi says in the Talmud in Chullin neither anywhere else in the Shas for that matter(Though the word "zarnich" which is a substance of black color, does appear.) The commentator on the Aruch, who is another Rishon points this out under the entry "gir", while the Aruch himself says that gir is just like "sid" which is lime or whitewash. Though, Rashi himself repeats this "gir vezarnich" in his commentary on Bechorot 19a as he refers to the same Gemara in Chullin. What seems to be the case is that Rashi had a version of the Talmud which is not in our possession today and since it was not seen by others, Rashi's reference cannot be explored or contested. What further complicates the issue is that Rashi on Avoda Zara 54a explains gir as frost which is white and in his commentary to Daniel 5:5 explains gir as being the lime painted on the walls and refers to the pasuk in Yeshayahu which you brought at the start. In any event, the Ramban in Shemot categorically disagrees with Rashi and says that gir is white and it is used to whitewash walls. However,though I am no chemist or geologist it is a known fact, that limestone does turn black in certain common environmental conditions. All the best