They do not make you stupid!

However, taste has been somewhat tackled in regards to precious metals and crystals with a general pattern of association that relates taste more to perception of a food rather than a one to one correlation. Hence, gems and precious metals are to kran as sweets are to us. It is not so much that these things are sweet but they fit that niche in the Kran diet.
In one of the books I wrote for the Stonehead reading shelf, I have proposed my own such system of organizing what Kran eat:
Crystals
Acid - Metal
Sand - Coal - Stone
So sand (silica), coal (carbon), and stone(various mixed minerals) are the staple foods while acid and metal introduce further needed "nutrients" such as pure elements and acid which is needed to keep their stomach acid replenished. At the top are crystals which aren't necessary but taste "good" and provide aesthetic appeal, see the kran books on that last part.
However, adding magic throws in an extra loop that has not been addressed. However, deciding what is what would actually be helped along by understanding gem enchanting better than I do. I'm not even sure what the different kinds of enchanted gems are.