It is hard to say. It is poetic language. Tolkien is trying to capture an idea or feeling he has. The idea is not easy to describe in plain English, so he tries to evoke feelings by is choice of words. "Silver glass" is not a common phrase. A mirror can be "silvered glass," which is glass with a layer of silver behind it. I don't think "mirror" makes sense here, but see my final remark. Here are my ideas. They might be right, they might be wrong. Gandalf is talking about death and the afterlife. Gandalf, and almost certainly Tolkien, believed in an afterlife. Death seems negative and depressing and "grey" from the point of view of "this world." If you believe that death is a voyage from "this world" to a better world, you could see death as positive and bright and wonderful. "Grey" and "silver" are similar, but silver is brighter. "Rain" and "glass" are similar, but glass is clearer. Imagine you are emerging from a patch of fog. From within the fog, the fog looks "grey." As you begin to emerge, the fog brightens and begins to look "silvery." By convention, we call rain clouds--"nimbus clouds" in technical terminology grey. There is a saying in English, "every cloud as a silver lining." So, "silver" is a word that is often used to describe a bright patch of grey within a darker grey. There might be an echo of a famous phrase from the Bible. The writer was speaking about death in a positive way. He wrote "For now we see through a glass, darkly; but then face to face" (traditional English translation), or "For now we see only a reflection as in a mirror; then we shall see face to face." So both here, and in Tolkien, we have an idea of emerging from a place where our vision is dark and indistinct into a place where everything is bright and clear.

Silver 'glass', not 'grass'. I think it is a poetic way of saying that during life we cannot know what awaits us after death - we can only see 'a grey rain curtain' - i.e what lies beyond is obscured. But after death that grey impenetrable curtain changes to 'silver glass', i.e. a clear, shining window through which we can clearly see.

Hello, Nikita. "Silver glass" is meant to describe a looking glass, which is more commonly referred to as a mirror. Gandalf is poetically telling Pippin that when we die, the great mysteries of life are made as clear to us as if we were looking into a mirror. The next part of the journey will be revealed to us when we die. I hope this explanation helps you!