Well... Science mode: ON
You know how gems are essentially crystalline structures of different minerals and compounds. Based on this, and how those minerals are present in the structure and flavor of actual food (iron, for example, zinc, calcium...), and medicines or other substances that we ingest, willing or accidentally, we can have a slight idea of how do gems and enchanted gems taste like. There are other factors that we can take into account, such as: crystallization, hardness, malleability, ductillity...
Let's take the example of iron: iron is present in our blood. Have you ever cut your finger, and instinctively sucked on it to try and close the wound? That metallic taste would be the iron present in your blood cells, or rather the ferrous oxide that transports the oxygen from your lungs to the rest of your body. So, Kran meals with predomination of iron elements would taste, well, like iron. What an iron-y.
Copper, and concretely cuprous oxide, a green patina created on the surface of copper and bronze objects, can be rather dangerous and poisonous. It's hard to tell how it could taste. However, it's demonstrated that soils with cuprous cummulations do feel crumblier than others, as happens with calcium (I think, correct me if I'm wrong). It would be like dry cookie dough, with an earthy taste to it, for Kran.
Any acid would feel (actually "taste") acid. But if you were to chug on a bottle of sulfuric or sulfhidric acid, you'd probably melt away your tongue, pharynge, esophagus... Basically, you'd die. But for a Kran it wouldn't be more uncomfortable than chewing on a lemon. Nitric salts, and other salts, would taste... salty. Surely, one of those salts could melt your stomach, but they'd still be salt. There's a difference between mineral and organic salts: when a mineral acid reacts with a mineral base, it will form a strong salt (like niter, sodium chloride); and when an organic acid reacts with an organic base, it will create a mild salt (tartaric salts, present in wine).
Kran are silicon-based organisms, so they are designed to eat these kind of things, and carbon-based dishes will have no effect on them, since they don't have a way to process it, because they don't need it. Coal-based dishes would taste like wood, or graphite, to them. If you've ever chewed on a small branch (like with natural liquorice root), you'd know, after the main flavor has faded, how wood tastes. Even a popsicle stick can help you. A pencil lead could be useful as well. Zinc is also present in Kran gastronomy. As Nolthrir eat seafood and fish, so can Kran. Only they eat it straight from the source. Zinc would be the seafood of the Kran, even being as soft (in terms of kran strength) as seafood is for us.
Let's move on to gems. Most, if not all of them, would feel crumbly, hard, crunchy, and salty. They are crystallized minerals and oxides, and their taste would be based on what elements do they have in them. Currently we have the following minerals: Diamonds, emeralds, rubies, sapphires, opals, amethysts, amber (albeit amber being from organic procedence, it's considered a gem in terms of jewelry), turquoise, and onyx, and I think I didn't forget any. Some of them you can't find easily, if it isn't through a quest, but the first four (diamond, ruby, emerald, sapphire) are the ones that are used in Kran cuisine.
Questionability of crumbling diamond to powder in a mortar aside, diamond is a carbon-based gem. If you've ever chewed on a pencil, and got to the lead, you'd know how it could taste: diamond is simply graphite, with a layered structure reorganisated into crystals, by means of pressure and temperature. The red color in rubies is due to the presence of aluminium (lumium) and cromium in them. Go chew on tin foil: that's what a ruby would taste like, maybe have a slight hint of iron oxide. Sapphires, being from the same family, would taste the same. Emeralds are a silicate: they'd have a sandy, earthy taste to them, as well as a hint of ruby or sapphire (since they also have aluminium-lumium in them).
As for the precious metals, you know, gold, platinum, silver, yadda yadda... It's hard to know what does gold taste like, even though some people who have eaten gold foil -those lucky patrons of the high-end restaurants where the Hollywood jet-set goes- claim it doesn't taste like anything special. They possibly have the best taste of all for Kran, though, given how scarce and rarely found they (supposedly) are.
I'm surely leaving out some other elements, but I think you get the idea clear. Investigate on your own! It's the best way to learn about the world surrounding you.
