While light-based attacks such as energy blasts or laser beams are common in fiction, they are often portrayed in an unrealistic manner. On VS Battles Wiki, they are only accepted to move at the speed of light under certain circumstances. Typically, lasers and other light-based attacks are only...
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*It is stated to be composed/consisting of photons or light itself, again by a reliable source.
Gems are stated to be comprised of light so many times in the series its not even funny, with many of these descriptions being made by characters with significant scientific knowledge such as Pearl and Peridot, with further evidence such as Gems being unable to move faster than light due to their bodies physically lagging behind them, their bodies not showing up on x-rays, and the formation of a Gem's body giving off heat rather than direct force supporting this.
*The beam diffuses in a reasonably realistic way or reflects off a material that it can be expected to, such as a non-magical mirror.
Gems have regularly used light projected from their gemstones as flashlights and the light used by Gems reflects off of mirrors
*The beam is called lightspeed by reliable sources.
While the beam itself isnt called lightspeed, the light used by Gems actually is, indirectly at least, by Pearl in Adventures in Light Distortion
*It has its origin at a realistic source of light, such as a camera.
In the case of SU, lasers coming from Gems is a realistic source due to the aforementioned fact that Gems being made of and utilizing light is constantly drilled into your head since the beginning of the series.
In addition, the beams dont curve or shift direction, they travel in straight lines, and they dont explode or cause visible force damage to the environment (lasers causing physical damage tends to be a big indicator of them not being true light so these lasers not doing so supports them being light further).
So yeah, theres ample evidence that it fits the criteria of lightspeed