Board Thread:Questions and Answers/@comment-7657843-20150504102237/@comment-1298029-20150510191730

As far as we know. None have ever been shown in more than 2. The Tesseract has the cube form.

Digression:

"tesseract" is actually a historical name for the n=4 hypercube, or 4-cube, the 4-dimensional (spatial, no "time dimension" here) equivalent of a cube; to construct, take a point, select a scalar (positive real number) a, select any direction in space, then move the point in that direction by the distance a; you get a line of length a as the trajectory of that point through space; take this line, select any dicretion perpendicular (90 degrees angle) to the first direction chosen, move it by a in that direction, you get a square of side a as the trajectory of the line; take square, select a direction (in a 3d space there will only be one axis remaining, restricting your choice to forward and back on that axis; in a general n-dimensional space there will, generally speaking, be no such restriction), move square by a in that direction; you get cube of edge a as the trajectory. You will notice that a point is a zero-dimensional object, a line is a 1-dimensional object, a square is 2-d, and a cube is 3-d.

Although the space we inhabit has only 3 dimensions, most mathematics, including geometry, continues working the same way in an arbitrary number of dimensions (4, 10, a million, whatever so long as it's a finite positive integer; not sure about infinite number of dimensions, you'd have to ask a mathematician). Hence, if you take the cube from this example and move it by a in a direction perpendicular to all previously chosen directions (this is impossible in a 3d space, hence we're unable to imagine it or draw it in the traditional way), you get the 4-dimensional equivalent of a cube, a 4-hypercube or tesseract (from Ancient Greek tessera, the word "four"). Interestingly, the movie Tesseract is just a regular cube (although a tesseract's cell-first parallel projection into 3-space [= 3-dimensional space, such as e.g. the one we live in] is a cube, so there's that). The process outlined above can be repeated ad infinitum, giving higher-order hypercubes (5-cube or penteract, 6-cube or hexeract, 7-cube or hepteract, and so on). (Again, you'd have to ask a professional as to what happens when the number of dimensions is infinite.) The mathematics of such bodies does not change in any special way between 3 or less and 4 or more dimensions. In fact, a set of linear equations describing a 4d geometrical object can be simple enough to solve in high-school mathematics.

End of digression. Sorry for the off-topic. :)

The Aether has the gas-dust (for want of a better word) form. However, the Scepter seems to have been a physical container rather than an alternative form. Then again, maybe the cube is too (it's shown shattering in Thor's vision in the pool). I'm pretty sure the Orb is just a lockbox for the Power gem, not some transfigured part of it. I'm not sure whether the yellow gem as it appears on Vision's forehead is the "primordial" (crystal-like) form or a transformed appearance.

The Aether appears to be able to change into a form resembling a pile of glass shards (when the mist form was "overloaded" by Thor's electric arcs), but that may just be a "phase change" of the normal fluid rather than a third form. It is seen immediately afterwards reforming into the mist, much like the T-1000 Terminator in Terminator 2 reformed into the "quicksilver" (no relation to the AoU hero) after being frozen (by liquid nitrogen, I think) and physically shattered.