Luminescent molecular aggregates, called J-aggregates, is very interesting example of excitonic molecular nanoclusters. Their structure could be different and is strongly dependent on microenviromentment. In some cases fiber-like J-aggregates are agglomerated and transformed into light-guiding microcrystalites. On the picture a combination of fiber-like J-aggregates (yellow) and light-guiding microcrystallites (red) is shown.