§1. According to the traditional practice of the Church, the table of a fixed altar is to be of stone, and indeed of a single natural stone. Nevertheless, another worthy and solid material can also be used in the judgment of the conference of bishops. The supports or base, however, can be made of any material.
§2. A movable altar can be constructed of any solid material suitable for liturgical use.
Source: Code of Canon Law (Vatican.va)