Before the Second Vatican Council, Mass could only lawfully be celebrated on a properly consecrated altar. This consecration was carried out by a bishop, and involved specially blessed "Gregorian Water," anointings and ceremonies. The relics of at least two saints, at least one of which had to be a martyr, were inserted in a cavity in the altar which was then sealed, a practice that was meant to recall the use of martyrs' tombs as places of Eucharistic celebration during the persecutions of the Church in the first through third centuries. Also in the cavity were sealed documents relating to the altar's consecration. The tabletop of the altar, the "mensa," had to be of a single piece of natural stone (almost always marble). Its supports had to be attached to the mensa. If contact was later broken even only momentarily (for instance, if the top was lifted off for some reason), the altar lost its consecration. Every altar had to have a "title" or "titulus" in Latin. This could be The Holy Trinity or one of its Persons; a title or mystery of Christ's life (Christ the Good Shepherd; the Holy Cross); Mary in one of her titles (Mother of Christ; Our Lady of Good Counsel); or a canonized saint. The main altar of a church had to have the same title as the church itself (for instance, there are many "side altars" in St. Patrick's Cathedral in Manhattan, but the "high altar" in the center is dedicated to St. Patrick). This reflected the idea that the altar was the key element, and the church was built to house it, as opposed to the church being built and simply supplied with an altar as part of its furniture.
Obviously, these regulations would have made it impossible to celebrate Mass anywhere but inside of a Roman Catholic Church. To provide for other circumstances—for chaplains of everything from military to Boy Scout units, for priests while traveling alone, for missionaries, or for large outdoor celebrations of Mass on pilgrimages, just to name a few situations—"portable altars," popularly called "altar stones," were used. These are usually blocks of marble either square or rectangular, often about 6 inches by 9 inches or so and an inch deep, and are consecrated the same way as altars described above. A priest with a field kit could simply place this stone on any available surface (the tailgate of a Jeep, for instance, or the stump of a log at a campground) to celebrate Mass, or it could be inserted in a flat frame built into the surface of a wooden altar. Many Roman Catholic schools, for instance, had a full-sized, decoratively carved wooden altar (which, being wood, could not be consecrated) in their gym or auditorium that could be taken out and set up for temporary quarters for Mass, with an altar stone placed in the "mensa" space.
The privilege of using a portable altar was not automatically conferred on any priest. Cardinals and bishops normally had such rights under canon law, but other priests had to be given specific permission—this was, however, easily and widely obtained.
Today, a consecrated altar is no longer necessary for the lawful celebration of Mass, so priests offering Mass in the field or schools or elsewhere may simply use any table. Parish churches and chapels often have wooden altars today, which may be blessed (as opposed to consecrated). Parish churches and cathedrals should have a consecrated altar, however, still made of stone, though the ceremonies for the consecration are somewhat simplified. Side altars, once common in churches because priests could not normally concelebrate before Vatican II and so had to offer Mass individually, are now frowned upon.
That an altar be built of stone goes back to the Bible. For example, Elijah built his altar of twelve stones: