Money, money, money!

Spoiler warning: while this post doesn't spoil anything about the plot, it does contain information that the characters couldn't know and that could possibly be immersion breaking.

What would be appropriate money system for the Lightless? In fantasy genre, gold is used to the exclusion of other options, with silver and copper filling for smaller units. But since most people don't use light at all except for engineers and magic-users who need to see the schematics(reading by touch is all fine and well, but it doesn't work when pictures are used), gold wouldn't really fit. After all, it was used not only for it's value, but because it's beautiful to look at and difficult to forge. But you can't sense gold's beauty without light, and the sound, weight and feel would be easier to forge. While gold may be rare, since it isn't really very useful(what is gold useful apart from electronics? False teeth? I can't really think of anything) and doesn't have aesthetic value without light, I really have to look elsewhere.

What I really need is something that either feels or sound unique. Shells of rare mollusks? Doesn't really work in day-to-day life. While I can imagine them being collected from several beaches or even a single beach perhaps only during a certain time of year and they could certainly feel and sound pleasant, they're either too large and cumbersome or too brittle.

But...Suppose there was such a rare species with easily recognizable feel and sound. Perhaps it might even have been used as a currency in the past. I'm sure the shells could have been replicated in metal. It would have to be a hard metal, otherwise they would still be damaged easily. It would also have to sound pleasant and the exact sound would have to be difficult to reproduce. Gold is obviously out of question because it's too soft at least as a pure metal. Besides, I don't want to replicate the clichee.

The shells are beautifully engraved with text identifying the mint, kingdom where they were made, year and value. They are made from silver-palladium alloy the exact recipe of which is a secret. While some forgers in the past have identified and replicated the alloy correctly, none of them has ever been able to replicate the exact sound the shells create. The secret of the sound is a small bubble of mercury in the middle of the shell. The sound is thought to be impossible to replicate exactly and most people handling money daily are well-practiced in identifying the sound of a forgery. The shells come in several variations: the smallest of the is half-inch from top to bottom and produces a sound near the high-end of the hearing spectrum. It's called bweeper. Old people sometimes have trouble identifying them correctly. The second smallest is a full inch long and is five times as valuable as the smallest one. It is called whistle. The third one is inch and a quarter long and worth ten bweepers. It's called key, and it's sound is often used to tune instruments.The fourth is the least valuable double-shell: it is made of two 3/4 inches long shells joined in the middle. It is worth twenty five times bweepers and it's named double-whistle. The second double-shell is made of two 9/8 of an inch long shells, worth two double-whistles and called double-key. The last double-shell, throne, is 11/8 of an inch long and it's worth twenty double-whistles. Numbers are usually called in bweepers and double-whistles or, when dealing with large sums, in thrones.