The following was posted to Essential's official Twitter account last night:

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"We've been working on a new device to reframe your perspective on mobile. It's now in early testing with our team outside the lab. Look forward to sharing more in the near future! #ProjectGEM"
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Additional photos were tweeted by Andy Rubin himself:

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This new device seems to have caught the Internet off-guard; all that's been leaked prior to this are a few snippets of code on XDA. There is, however, plenty of speculation at the referring links below.

via Android Police, The Verge