Vaseline Glass was primarily made before WWII and one of the primary ingredients used to make the glass was uranium. When placed under a black light (or ultraviolet rays), Vaseline glass will "glow" or fluoresce. The high energy emissions of electrons from the black light cause the uranium oxide particles in the Vaseline glass to become unstable thereby causing this fluorescence. The way we like to describe the effect is that it causes the piece of Vaseline glass to look like it has exploded in flames.

No other transparent antique glass will glow thus making Vaseline glass truly unique in the antique world as it can always be verified to be true Vaseline by using a black light. There is no other compound added to glass which will cause this effect except URANIUM. The yellow-green glow is characteristic of uranium-bearing glass and will also occur with custard and Burmese glass. The latter two glasses, however, are easily distinguished from Vaseline Glass in that they are not transparent.

Vaseline Glass can be difficult to detect by the human eye alone and even the best expert can be fooled. Only by testing with the black light will a piece be verifiable.

Vaseline Glass can be yellow, yellow-green, or green. Individual collectors each have their own preference as to the exact color of Vaseline glass collected. However, as glassmakers of the 1920's - 30's added iron to their uranium oxide dyes to put a green tint into their glass and although these pieces will flouresce under a black light, it is our opinion that Green Vaseline Glass (with no trace of yellow) is only a forerunner of Depression Glass rather than true Vaseline Glass.

The government confiscated all supplies of uranium during WWII and halted all production of Vaseline glass. Only in later years after nuclear testing, etc., was banned did the government ease restrictions somewhat and allow a very limited supply of uranium to glass companies in order to resume production of Vaseline glass.

As the uranium supplies are closely monitored by the government and tightly distributed between various glass companies, even more recent pieces of Vaseline glass are highly sought after by collectors as each is of a VERY LIMITED PRODUCTION.

How Safe Is Vaseline Glass In The Home?
Quoted from "Yellow-Green Vaseline - The Magic Glass" by Jay Glickman:

First of all the coloring agent used in glass is not the unstable and radioactive element uranium, but the more stable compound URANIUM DIOXIDE.  Secondly, this uranium compound is only 1% to 2% per volume of the glass. Finally, and most importantly, it is the medium, glass itself, which contributes to the safety of Vaseline Glass.
Glass, being a solid, is a barrier or shield to radiation. Glass is actually used in radiation rooms for such purposes.
The bottom line is that there is an insignificant amount of radiation emission from Vaseline glass roughly equivalent in distance to source as TV and microwave-oven emissions.