Maybe years back when the dungeon you're in was first explored tin was common in the area and this metal practically worthless, but as the mines ran dry it was necessary to bring the valued tin from distant lands. To bring this small treasure to town could mean some good gold for your party.
So what other apparently worthless pieces of treasure could a GM put in a dungeon?
Well there's salt for starters. A very valuable product for salting food. Medieval fantasy settings lack something we take for granted: refrigerators. So food needs to be salted to preserve itself. Armies marched on their stomachs and needed lots of preserved food. Salt can be a valuable item to find in a dungeon or as a treasure after defeating a hostile tribe of monsters.
Salt was traded for gold in the Sahara. Large caravans would take it from its mines in the middle of the desert to the sub-saharan cities and trade them there for gold.
Another metal you might want to put as treasure is quicksilver, now known as mercury. It is a liquid metal at room temperature and can be used in many things. One of which is amalgamation of gold, making it useful in gold extraction and processing. Mercury is very heavy though, as can be seen by the floating coin in the image below.
Mercury has another outstanding property. It is very toxic. So it can be useful in spell components and for not so honest activities. As its ore cinnabar it is very toxic to mine and process.
Pure elements like sodium can be very interesting as treasure too. Sodium doesn't naturally exist in pure form. It can be isolated through electrolysis or in the world of D&D through magic. Pure sodium in a jar full of oil can be easy to carry around, but take it out and place it in water and a monster comes to life.
Sodium is one of the elements of common salt which is found diluted in water in large amounts known as oceans. Isolated though, sodium becomes very exothermic in water (produces a lot of heat). The reaction produces the highly caustic sodium hydroxide (trust me I accidentally took a breath of it and it burned my nose and throat real bad) and the highly flammable and explosive hydrogen. When in water and in sufficiently large quantities sodium might melt into spheres and explode. So you've got heat, corrosive colorless and odorless gas and liquid and a very explosive gas. Could there possibly be something cooler than that? What better for a party to have at hand and a GM to surprise players with!
Sodium Hydroxide burn (2d8 HP + 1d6 per round until neutralized)
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