dnd 5e – How to determine the cost of ruby dust?

Unless your GM rules otherwise, you can use a real world analogy to crushed ruby, diamond powder.

Diamond powder of an appropriate grain size (around 50 microns) burns with a shower of sparks after ignition from a flame. Consequently, pyrotechnic compositions based on synthetic diamond powder can be prepared.

If you do want some effect from crushing gems in your game, you can apply some arguments below

The dust costs less

This quora post implies that a 1kg or 5000ct diamond costs $16,250,000.00.

Natural Diamond powder seems to cost $2/ct, so equating the two means 5000ct of natural diamond dust costs $10,000.

That’s a price drop of 1/1,625 for crushing gems into dust*.

So your 500gp ruby is now 500/1625 gp or 0.3gp. That’s the equivalent of 3sp if 1gp is 10sp.

So in the real world the price really gets crushed.

The dust costs more

In the real world however, you can’t use gem powder to cast spells, so the above argument won’t necessarily hold true. It’s down to the GM to decide if this is the case, that the labour in crushing gems increases the value in the same way purifying raw materials like iron ore into steel increases the value of the product.

It costs the same

It may be that your GM is not swayed by either argument, especially as it relies on real world analogies, or trying to understand D&D/fantasy economies. The simplest solution is to have the two cost the same, either by GM fiat, or some balancing of the above two arguments.

*Note, that it’s possible that diamond dust has more utility than ruby dust. Or less.

It’s quite likely these arguments about how economics work in D&D are unconvincing, largely because the economy in D&D doesn’t make much sense if you look too closely, and even if it did there are too many differences to make proper analogies.