What was forbidden was

Knowledge - it was forbidden for them to eat the fruit of the tree of knowledge of good & evil

As tangible things go, it makes me think of a

library, or school/learning

pride/ego?

hmm, it also reminds me of

a kid's music box

I believe the rule is

Square it & add it.

2, 2^2, 2^2 +2, 3^3, 3^3 +3, 4^4, 4^4 +4, 5^5, 5^5 +5, 6^6, 6^6 +6, 7^7, 7^7 +7

your dreams

Admittedly this is not *quite* correct, but if you take into account some vernacular language it could also be Kinetic & Potential (energy) states.

And don't forget Plasma!