Board Thread:New on Marvel Cinematic Universe Wiki/@comment-1509585-20140804183803/@comment-1652820-20140805001102

The most convincing argument that it's another Infinity Stone is that it couldn't be powered by the Tesseract since Loki had it before coming through the portal. Thanos didn't have the Tesseract, so he couldn't use the Tesseract to power it before giving it to Loki. However, it's also possible that it's not another Infinity Stone but merely a gem that has been POWERED by an Infinity Stone. Meaning that Thanos might have the real Mind Stone in his possession, and merely charged up the scepter with a small portion of its energy (the way the Tesseract can charge weapons).

The most convincing argument that it's NOT another Infinity Stone or even related to any Infinity Stone other than the Tesseract is that it glows blue, like the Tesseract. In the comics, each Infinity Gem is a different color and they appear to be following that pattern in the MCU.