I have to further add that the OP's question also included:

"what percentage of these grains are effective to record a picture"

This would make absolutely no sense if "grain" were to mean composite effect of graininess when looking at the final image. So it's absolutely clear from the context that these grains are crystals in OP's terminology of choice.

Also, the plural form gives away the meaning.

As for the percentage, I think it's quite near to 100% for today's films. Of course, if you don't use the complete exposure latitude of the film, some of the smallest are not "used", but they COULD be used given enough exposure.