If you are playing with the good old "10 coins equals 1 pound" default in your setting, carrying around treasure, even gold or platinum, is damn near impossible at high amounts (at least w/o bags of holding or portable holes).
I know when I think of gems in D&D settings, diamonds, rubies, sapphires and the like come to mind, although it is more likely the party will come across semi precious stones, such as the tumbled quartz and other stones pictures above.
Even if the gemstone were worth just one gold piece, it's still smaller and weighs significantly less than it's corresponding coinage. Maybe there is a society that avoids metal coins all together, and uses quartz of different sizes and colors for monetary transactions.
Broken semi-precious stones would, however, be next to worthless, whereas melted or bent coins still have the value of the metal that composes it.
The Minotaur for Old-School Essentials
*Requirements:* Minimum STR 9, Minimum CON 9
*Prime Requisite:* STR and CON
*Hit Dice:* 1d10
*Maximum Level:* 8
*Armor:* Leather, including shield...