Just out of curiosity, why would you need that? The very dust of the earth, from which all saints are made, is a relic of the creation of God. Why should some molecules of dirt have more value than others? Why should we venerate the dust of the earth? Should we not simply praise God for the work of the martyrs (which is the work of God anyway) and give all the glory and honor to God? Isn't that what THEY would want?
To mirror the souls of the saints under the Heavenly Altar as recorded in the Apocalypse of St. John, and thus as a reminder of their prayers.
The Church has done this since the earliest days: in the days of the persecution by the Roman pagans, the tombs of martyrs in the catacombs serves as the holy table.