Ordinary glass is fine.

Newton's rings are an interference pattern created when the contact between the negative and glass isn't perfect. The problem can be avoided by using special glass, by adding a fine powder between the glass and the negative (although that's probably not a great idea if you are making contact prints), or by making sure that the glass applies uniform pressure across the 8x10 print surface. The latter is the most common solution, and either involves using glass that is thick enough that the weight of the glass completely flattens the negative, or by using a contact printer with back springs to flatten the sandwich of glass, negative and printing paper.