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Far fetched to some, but recreating the picture within Minecraft shows that there's no full block where the sliver of grey is located. Possibly a wall, but I'd like to think it's something a bit more fun.
Here's a little run-down and recreation of the tweet.
Mojang has revealed some secrets within seemingly innocuous tweets. The Structure Blocks were revealed first by being hidden in a screenshot!
The quickest way to my heart is with a smile.
Oh, and a white-oak stake.
There is already an active thread discussing the 1.14 information from Mojang:
Please use that thread for this discussion.