One more solution

public static int gemstones(List<String> arr) {
    Set<Set<String>> countset = arr.stream().map(s -> s.split("")).map(Arrays::stream)
            .map(stream -> {
                Set<String> set = new HashSet<>();
                stream.forEach(set::add);
                return set;
            }).collect(Collectors.toSet());
    Iterator<Set<String>> iterator = countset.iterator();
    Set<String> firstSet = iterator.next();
    while(iterator.hasNext()) {
        Set<String> set = iterator.next();
        firstSet.retainAll(set);
    }
    return firstSet.size();
}