An extremely rare refractory metal, rhenium is most often found in superalloys, alloys that are extremely durable and resist deformation at high temperatures, such as those used jet turbine engines.
One of the more unique uses for rhenium, however, has been in dart barrels. Rhenium's use in darts began not because of its heat resistance or strength, but because of its density.
Rhenium is amongst the most dense metal elements (it is only surpassed in weight by iridium and platinum), which means that rhenium dart barrels have the highest mass to volume ratio available of any dart. (Actually the barrels are made of a tungsten-rhenium alloy containing about 5% rhenium).
The increased density allows for a sleeker, thinner design that theoretically - although I don't know that it would my game much - leaves more space to hit three treble 20s.
Image: Rhenium-tungsten alloy darts. Copyright © A180 Darts Ltd. www.a180.co.uk