When thrown, they could turn a given surface into a reflecting one, which means that it will bounce spells while it normally does not (doors).
If thrown at a monster it could act in the same way, the monster would reflect (for some limited time perhaps) elemental spells back in the direction they came from, or a random direction.

If you dip your shield in it, you can reflect hostile spells as well (even your own), until it wears off, perhaps with similar mechanics to poisoning a weapon for a single poisoning blow.
A single bolt would be reflected but the reflective layer would be destroyed in the process.

If the random reflection happens in a corridor, there's a chance the bolt will bounce between two walls and kill the PC in the middle (or deal damage similarly to already existing cause of innumerable YASDs, mine included)
Perhaps there should be a chance to find items already dipped in the substance - those would have to be examined before equipping, to determine their status.

The chance/amount of spells reflected could be dependent on how many items have been dipped (2 shields, armor, maybe gloves/helmet). The direction would be random because of so many irregular and differently angled surfaces on the worn items.
At the same time it would be impossible to sneak around (stealth skill) due to excessive reflections and glittering.

Since reflective substance would most likely be some sort of naturally occurring liquid metal, like mercury, it would amplify lightning/shock damage instead of reflecting it, due to higher electrical conductance.
That way you could throw it at a monster and then electrocute it for extra damage maybe?

Casting light spell in dark areas with one or more items worn with the substance on them would increase the radius of the spell due to reflections; similarly casting darkness would reduce its effectivity with such items around.
I suppose there are other potential uses only limited by imagination. What do you think?