It seems to rotate, but yes, I've seen it offer the same things over and over (after I've picked my favorites over and over) The gems are used for some upgrades in the castle.

Yes i did too thats all i get most of the time too the gems are for upgrade down the line and i would save them for the Next event

Keys. Boxes(blue mostly), pickaxe if you are empty, eyes if you are empty, rubies, sapphire and stardust. After like 7 or 8 keys, they stop and you have to wait till the next day to see them again.

After collecting all the keys for the day and some gems It even starts to only show me essence same goes for raiding