They drop from all ores / mobs. We can make them only drop from greater, however.
This is really dependent on what direction you want this trading stuff to go in.
If you want the really good stuff (fortune, silktouch epic tools, etc) to be the rewards, the higher difficulty of having to mine them from greater ores would be a good checkpoint to stop the market from getting swamped.
The other end of the spectrum is leaving it dropping from all ores. This will *potentially* allow a much greater amount of the symbols into the economy, destabilizing and lowering the worth of this new currency faster.
In my opinion, and with a much greater amount of testing ahead of me, I think that dropping from greater ores and mobs should be the only way to get the symbols. The description of the symbol reads something along the lines of "with great determination" or something*. You should be determined to find the greater ores and be rewarded for mining them. Getting these rares from common ores like coal and iron kind of defeats the purpose. Isn't that why you removed the chance for relic Vs and codices from minor and lessor ore?
However, if you settle on limiting the drops to mobs and greater ores, you should probably slightly increase the drop rate. Half of the drop chance that Large fragments, have may be the right level.
-Will continue adding progress and feedback as I mine.