Through many time periods things have had value due to their usefulness or their appearance and/or rarity such as precious metals like gold and silver and coins made from them, precious stones like diamonds and rubies, or even shaped metals like Chinese knife money. However one thing common across many time periods was that materials that are very common like rocks or dirt have had very little value. But what about a material that is both common, innately useful, and pretty to look at.
In the world I'm writing of there is a material called nautilite that is as strong as bronze and has the appearance of white iridescence similar to nacre when light is shone upon its surface. This magical material is what a lot of molluscs have their shell composed of and when their shells as they so commonly do wash up on the shoreline where they shine in their beautiful iridescence they are very easy to spot.
These shells can be made into many things such as jewelry due to their beauty or even have holes drilled into them and be made into an armor suit made of overlapping shells which is comparable to the bronze armor already present(my setting has iron working being very rare and steel is nearly nonexistent with only a few pieces of steel being present and given legendary status). Larger shells can even have a leather and felt liner set into them to create a beautiful and very protective helmet.
However this does present a problem for rulers and that is how do they make sure they have a stable economy that doesn't collapse when someone brings in thousands of these nautilite shells that they found which can be used for many things due their material strength being comparable to bronze and their innate beauty?
EDIT: Some additional info is that Nautilite can't be melted down otherwise it loses its magical properties like durability and appearance when heated in a sufficiently hot enough fire(2000 degree fahrenheit). Nautilite can't be reshaped like bronze and if subjected to great enough force will begin to crack and chip. Bronze(or any sufficiently hard metal) is also needed to drill through Nautilite because a stone drill will wear before it even begins to scratch it.