Why does the wood of the Black Locust tree fluoresce when exposed to ultra violet light?

I know that normal fluorescence is caused by the inclusion of minerals such as uranium salts. But this is most commonly seen in items such as yellow (Vaseline Gl) gl where the color yellow comes from the uranium. Now I received a same of Black Locust lumber from a friend and to my amazement it does fluoresce. Another of its properties is that is an extremely hard wood and rot resistant up to 70 years. If anyone can answer my questions or give me some suggestions of where or how to search, I will appreciate it. Thanks! Sheri