Altar Stones

I am researching an altar stone that has recently come to light in Kildare. Mass, up to the Second Vatican Council, had to be celebrated on a properly consecrated altar. This altar contained an "altar stone" which incorporated the relics of two saints. So what has became of all the altar stones in Catholic churches that have since been altered? Many of the side altars in churches have been removed; where are the Altar Stones of those altars? Anybody know?