According to the Constitution on the Sacred Liturgy (nos. 328-29), all vessels should be made of precious metal; or, in the case of the United States, a durable material such as an ebony or hard wood. The reason why glass is discouraged is because glass can break very easily, and the sacred vessel should not be so fragile. However, despite the fact that glass is not mentioned as a suitable material for the Chalice, you will see some churches using glass for the sacred vessels.