Spheres...I find them very interesting and at the same time odd. Instead of metal coins, the inhabitants of Roshar use gemstones found at the center of a glass marble, i.e. spheres. It makes a lot of sense if you consider the existence of soulcasters.
Soulcasting can transmute any material into another material except true gemstones. So, if you had coins made of metal, you could basically print your own money with a soulcaster. However, because of the inability to create true gems via soulcasting gems are still a rare item.
What makes even more sense is that there is a natural economical sink for gems. Gems power soulcaster and are consumed by the act of soulcasting. So, there is always need for more gems. So, gems are valuable.
So, gems as a monetary system are segregated based on use and rareness. Some gems are more valuable than others because of their use in soulcasting; e.g. emeralds are valuable because you use them to make food.
However, the issue that I find quirky is how the gems are managed. Naturally, handling lots of gems, especially small "chips" of gems which serve as the lowest denomination would be difficult. So, Brandon's answer are glass spheres. Gems are embedded in the equivalent of a clear glass marble, colloquially called a "sphere" much the same way we say "dollar".
I just can't for the life of me think that a glass sphere would be the best way to manage money. I will assume for the sake of simplicity that the glass sphere are by some means very robust and do not easily break. But, as anyone who has handled marbles as a kid...they are a real pain to handle. They roll everywhere. You can't stack them. They always compact into a clinking noisy spherical mass.
So, my point is, I would have picked a different shape of glass. Instead of a sphere, perhaps a rectangular prism. This would be an improvement, to me as the reader at least, in a few ways. First, you could stack them. Second they would still function as a light source but you could use them in something akin to a candle holder versus vases. Third, you could have a more flexible money system.
How exactly would this be a more flexible money system? Easy, each stick could hold up to say 5 chips or broams. This would allow for a base 5 system of counting versus a base 1. It would greatly reduce the number of spheres that needed to be carried.
So, the sphere system I just find quirky. Mostly because it seems so impractical. Visually though it would be arresting to see a glass sphere with a glowing center.
Image Source: Scanned Cover