They are a currency being added for the upcoming system of "Crown Crates".
Crown Crates are boxes you buy for 400 Crowns (if they do not change the price before release). It comes with some crown consumables and (almost always) one collectible. The collectible can be one of a list of collectibles that will be available to be previewed in a separated section of the Crown Store.
There are four tiers of collectibles. When you get a collectible you already have, you get Crown Gems instead. The amount of Crown Gems depend on the tier of the collectible you drop. On the PTS, you would get back a little under 1/3 of the amount of gems needed to buy a collectible of that same tier. ZOS said they are thinking of making consumables be traded for Crown Gems too.
We don't know yet when Crown Crates are coming exactly.
Just so that everyone knows, my Altmer still can't have black hair. About a dozen of Altmer NPCs in the game have black hair. Just saying.
Meet my characters:
Command: Do the thing.
Zadarri, Khajiit Fist of Thalmor: The thing was done, as commanded. Durza gra-Maghul, Orc blacksmith: The thing was done perfectly, in the most efficient way. Tegwen, Bosmer troublemaker: You can't prove I didn't do the thing. Sings-Many-Songs, Argonian fisher: Sure, I'll do the thing... Eventually. Maybe. Aerindel, Altmer stormcaller: After extensive research, I've come to the conclusion that doing the thing would be a waste of resources. Liliel, Dunmer pyromancer: Aerindel said I shouldn't do the thing. Something about "resources". Gyda Snowcaller, Nord cryomancer: I will find a way to do it that won't waste resources and make Aerindel proud of me. Beatrice Leoriane, Breton vampire: I persuaded someone else into doing the thing. You are welcome, dear. Sahima, Redguard performer: Doing the thing sounds awfully unpleasant and really not my problem. Ellaria Valerius, Imperial priestess: I'll pray to the Eight for the thing to be done, if it is Their will.
Crown Gems have the same purpose as crowns, to lull you into a false sense of not really spending money using a fake currency rather than just letting you buy items directly with a real money value....(all money is technically fake and we are the one's stupid enough to buy into giving paper with ink on it value but that is another discussion for another day :P )
The whole point is to make you feel like your not really spending £ $ etc...
Crown Gems are extension of that kind of casino style gambling scam.... don't buy into it...
Ask yourself this, why have crowns or gems at all? Why not just sell the "mount" for £7.50 and the pet or whatever for $10...you have your answer...they can do it, they just choose not to because....
ZOS is adding gambling to the game for (exclusive?) new cosmetic items. You get gems if you get an item that you already own, and can collect them to buy an item directly from the gambling loot table.
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I thought gambling wasn't allowed due to the country's that play? I suggested something like this ages ago but not for the crown store and was told This.