// Usage: Block placeAt = findOpenBlock(chunk.getX() * 16 + randomX, chunk.getZ() * 16 + randomZ, world);
// randomX and randomZ are 0-15 values

   public static Block findOpenBlock(int x, int z, World world){
        Block current = world.getBlockAt(x,0,z);
        Block last = current;

        for(int y=1;y<world.getMaxHeight();y++){
            current = world.getBlockAt(x,y,z);
            if(current.getType()== Material.AIR && last.getType()==Material.STONE)
                break;
            last = current;
        }

        return current;
    }