The beauty of fireflies, which are an indicator of environmental health, will overwhelm you.
It sounds fine when I say it, but not sure it is grammatically correct. Does 'which' indicate the beauty of fireflies or the fireflies? Can I say that what the 'which' indicates is depending on the verb after it? (singular: the beauty of fireflies, plural: fireflies)
If it's wrong, is there any revision that I can make with the fewest changes?