I am probably not the most educated in precise terminology however I believe this is as you described a fissure/crack whereby an “oil on water” type phenomenon is being demonstrated possibly due to water or liquid inclusion creating a similar effect to an opal (ie. differential penetration and return of light producing rainbow colours). I have seen this in quite a few mineral specimens and quite often in the quartz family. I don’t believe this would be rare or desirable in peridot unless it was in a specimen in such a way to render it uniquely collectable. The feature would reduce likelihood of successful cutting/faceting as it could cleave along that plane.
I hope the above was of some help.
All the best,