@AnWulf - No, I'm not joking. Words only have a feeling such as the one you mentioned if you assign it to them. They are just words. I chose that word for its definition; it seemed quite fitting as a replacement for 'set in stone' as it is defined as: Unchanging over time or unable to be changed -- which is what I believe the OP was getting at when they were using the original adjectival phrase.

You may be correct that it isn't about syntax, but what I am saying is that I agree with Tom in that it would be much easier to avoid the problem altogether by using different words to get the same point across.

When you're choosing your verbs you have to pay attention to the plurality of the subject. The second half of the sentence without the conjunctive 'and' can stand as an independent clause; the subject of it is 'language rules' which is plural. According to subject-verb agreement rules, if the subject is plural, the verb form must match. The most simple solution to this problem is to follow Tom's advice and say:

"language rules set in stone are among them." using ARE and not IS because 'language rules' is plural. Whether or not it sounds better given the words involved is not the point. The correct answer is ARE because of the plurality of its subject. Tom is and has been correct the whole time.

Also @AnWulf - does your 'o' button not work?

Personally, I believe "language rules set in stone" is plural, simply because of the word 'rules'.
The true problem lies in the syntax.
I concur with Tom - the sentence flows better by changing it to "...language rules set in stone are among them."
Another idea might be to change the quantifier 'set in stone' to an adjective like 'immutable' or 'uncompromising' -- this makes it much clearer that the subject is plural and should be accompanied by 'are'.