Cool to hear this mentioned in the telethon! Its been something I've been thinking about, as its challenging to come up for with ideas for residue for a few of the elementals. Here are my ideas, FIRE ELEMENTAL This is the most obvious; Sulfurous Ash or Brimstone. EARTH ELEMENTAL / SAND ELEMENTAL Quartz or Quartz Sand. AIR ELEMENTAL Fulgurite. This is a mineral which forms when lightning strikes sand. 'Fulgur' comes from the Latin for 'lightning' and this is often referred to as 'petrified lightning.' The first photo I think is of fulgurite created experimentally, while the second two pieces are natural. Natural fulgurite is hard to find because its delicate and breaks apart, which is why you don't see as many branches. Interestingly, pieces of fulgurite have been found in Ancient Greek shrines to Zeus. The interesting part of this is that pieces of fulgurite can look like dead lightning bolts, WATER ELEMENTAL / ICE ELEMENTAL Gypsum. A mineral which as a compound contains water. Gypsum is deposited from lake and sea water and in hot springs, from volcanic vapors. It dissolves over time when placed in water. A runner-up here was Sepiolite, better known as Meerschaum, because it was thought to be hardened sea foam, but I thought it was an uninspiring material and not something that could be carried over to ice elementals.
For some reason, when I was reading about gypsum, I didn't look up other examples of minerals in the class of what are called 'evaporites.' Other possibilities in place of gypsum include borax and rock salt (halite). (Or simply plain salt might be a possibility).