The German Wikipedia really has a very good illustrated article about the Prussian Crown Jewels, which easily can be translated with Google Translate:
Wikipedia: Preußische Kronjuwelen.
More pictures here:
Wikimedia Commons: Prussian Crown JewelsAs can be seen there seems to be differing opinions about (and models of) exactly how diamond-studded the crown of Friedrich I was, but
a contemporary account from the Prussian Master of Ceremonies Johann von Besser tells that the frame was totally covered so that it appeared to be made of diamonds, not gold.
Correction of the WW2 disapperance story: The Prussian Crown Jewels were taken from the then Hohenzollern Museum in the Monbijou Palace in Berlin to Königsberg for safekeeping, then to a mine in Thuringia. From there the Americans brought them to Hesse (Friedrichshof?) before they were returned to the family and/or state. Somewhere along the way the Crowns of Wilhelm I and Augusta went missing along with the cheap nickel silver models of crowns for the German Emperor and Empress and the Prussian Nuptial Crown. The Hohenzollern Crown of Wilhelm II was found in the wall of a Westphalian village church!
I can't see whether the arched and unarched Electoral Caps of the Electorate of Brandenburg seen on the pre-WW2 photos have survived, but the arched one probably looked a lot like the Austrian Archducal Cap to the left and the unarched certainly looked like the Saxonian Electoral Cap to the left:

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