Kinda reminds me of the Purple crystal from Crash Bandicoot.... I love this!

 so simple and nice :D

Thank you! Yes, maybe I should have added more faces in the diamond and the effect would have been better. (No transparency limitation inside the diamond...)

 Although your entry may not be the best of the bunch technically, I surly cannot deny the appeal of it. I voted for you :)

 EXCELLENT! Excellent use of space!

Logical and effective!

 Nice idea!

I think it's a clever idea and a good animation. I like it!

Thanks! Yes, I think it isn't a very good animation, as Drazelic says I'd need use some transparency at both sides in some frames, but this is a challenge limitation (only transparency outside). And Cure is right too, maybe I had to colour some faces with intermediate colours, the highlights give the sensation of polished material. I thought that it vanish at the center of the frames because light rays are tangent to that face.

I'll try to improve this technique in the future.
(Thank you everyone and sorry for my bad english )

 Haha, this is a really creative idea, although after staring at it a while I see it as only half turning and then  reverting back to where it started. Good job giving the idea a go though, I would have lots of trouble creating that kind of pretend 3D effect XD Good luck in the challenge : )

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Hah, good idea)

 Unless the diamond's cross-section is a perfect circle and you're just doing some super celshading, the diamond is actually increasing and decreasing in diameter in a sine equation with regard to time.

In other words, it's not an actual physical object you have here, because the shape of the diamond isn't constant.

the briefly-flashing horizontal highlights are a little weird. they appear and vanish too abruptly, rather than changing as the diamond rotates.