I have had this idea for a while for what necrozma is based off of, earth. Here is the evidence that supports this.
1. Necrozma is called the prism Pokemon making a little suggestion to earth of where many crystal variants are found(may be bad evidence but t gets better)
2. Necrozma's new forms of Dusk mane and dawn wings are references to the times of day and the eclipses (Dusk mane to solar and dawn wings to solar)
3. (Can't say the same to dusk mane form) Dawn wings' shiny form may be a reference to the lunar eclipse when the moon gets to the umbra of the earth's shadow and is called the "Blood moon" and gives a red appearance as if it was covered in blood.
4. Necrozma is also titled as the reminisce of the ultra beasts. This may be referring to the dyeing of stars makes new stars and planets.
5. The blue in shiny necrozma may be a reference to the water that earth has.
There are some flaws to this theory as well
1. What does dusk mane and shiny dusk mane represent
2. what does ultra necrozma and its shiny represent.
3. Necrozma is never a rock type
4. There are many holes in the first point of evidence.
If there is something different you would like to suggest or some more evidence I may have missed or overlooked then please post it as an answer, I would love to see if this may be true.