Any ideas?

Noticed a beautiful necklace at my Nan's recently and we are curious as to what it consists of, it is very weighty c200grams by feel in hand. the beads are cut where the most beautiful colours are seen on the different ??facets??, they are red yellow, orange, blue. the gem is colourless however and when moved around the colours change on each of the facets. The beads are around 1cm in diameter.

Any ideas gratefully accepted,