I think we should definetly use a resource(CoMMMON sense=http://) that isn't too hard to find.  So that's why I chose glass.  Glass is sand, so maybe, just maybe, there could be glass replacing rare minerals like diamonds, emrald, amber, and silver.  maybe we can replace the resources from coins, maybe put it into glass and cover it in hard wax to prevent it from shattering.  And there's plenty of sand! It's always on the beaches!  but maybe when they create glass cups, they should cover the outside with hard wax that prevents itfrom falling and breaking.  So just take it into consideration that  maybe glass can be the next generations.  But sand isn't the best resource (probably).