Bag of air
It is a bag of air which has been zipped closed "for freshness". It is fuller than any bag like this you have ever seen. You could almost use it as a pillow, if it came with a case. These are often included in boxes to protect items for shipping to local shops, however; no one knows that really the price you pay is for these and NOT the other items in the box.
The percentages of each exact mineral in this particular bag are constantly being debated by heroes with nothing else to do or on a dare while at the local tavern with the promise of a free beer if they manage to guess it correctly on the first try. Despite the fact that no one has ever managed to guess the specific contents, heroes and heroines alike continue to try.