int amount = 0;
ArrayList<Location> stones = new ArrayList<Location>;

World w = Bukkit.getWorld("world");
for (Chunk c : w.getLoadedChunks()) {
int cx = c.getX() << 4;
int cz = c.getZ() << 4;
for (int x = cx; x < cx + 16; x++) {
for (int z = cz; z < cz + 16; z++) {
for (int y = 0; y < 128; y++) {
if (w.getBlockAt(x, y, z).getType() == Material.STONE) {
stone.add(w.getLocation());
amount++;
}
}
}