One of the unexpected "Wow!" factors in marble collecting is discovering that some vintage marbles glow yellow-green under ultraviolet light. These marbles owe their fluorescence to the presence of uranium glass, which is glass mixed with uranium oxide.
In the 1920s this glass was called vaseline glass because it resembled the color of the popular petroleum jelly sold at the time. Sometimes you can spot these marbles without using a black light, such as the two vaseline slags in the bottom of the group shown here. They have the pale yellowish-green color of vaseline glass which is a strong indicator that these marbles will fluoresce.
On the other hand, the Christsensen Agate "bloodie" marble in the center gives no indication whether it will glow under black light. Some do and some don't. In this case, it does, but not nearly so brightly as the surrounding marbles. In fact it is being overwhelmed by them, particularly the vibrant glow of the Akro Agate Co. lemonade oxblood marble at 11'o clock. Notice that its cousin at the top, the milky white oxblood, does not glow at all.
While this neat effect has limited value to identification purposes for collectors, it is still fun. Next time you score a big batch of new marbles, try to guess which ones will fluoresce and put them in a pile next to the rest. Then flip on the black light and be surprised.