Rev 21:19 

19 The foundations of the city’s wall were adorned with all kinds of precious stones. The first foundation was jasper; the second, sapphire; the third, chalcedony; the fourth, emerald;

precious stone – which are the good works of the saints, see 1 Cor 3:12. On these stones are the names of the Apostles, these are listed according to the shade of colour.