What is a verb for the meaning "To be identical"?
identical adjective
Things that are identical are exactly the same.
Collins
For instance, instead of saying "Diamonds are never identical", I can say:
Diamonds never [insert a verb here].
What is a verb for the meaning "To be identical"?
identical adjective
Things that are identical are exactly the same.
Collins
For instance, instead of saying "Diamonds are never identical", I can say:
Diamonds never [insert a verb here].
The verb match can be used intransitively for the meaning 'to be identical'. The definition of the intransitive verb match for this sense from OED is:
(of two or more things) to possess identical or complementary characteristics; to go or fit together (also with up).
Wiktionary provides a more succinct definition and a very relevant example:
match
(intransitive) To agree; to be equal; to correspond.
These two copies are supposed to be identical, but they don't match.
Although, it is not that usual or natural to say:
Diamonds never match.
A natural way to say is:
(Two) Diamonds are never alike.
Still, one can find usages with the structure 'Xs never match'. Here are two examples I've found:
My brother Otto had a sweater with the very same snowflakes. Absurd, our father said, —snowflakes never match. Each one has its own crystal network.
The Rags of Time: A Novel by Maureen Howard
The DNA might be identical, but the fingerprints never match. In more than a century of use, in every country on earth, no two people have ever been shown to share the same fingerprint.
A Question of Evidence: The Casebook of Great Forensic Controversies, from Napoleon to O. J. by Colin Evans
Additionally, the verb conform can be used intransitively for this meaning as well; but it doesn't work for your example sentence in colloquial usage. Perhaps, it can be used in a technical context in mathematics, geometry etc. Merriam-Webster definition of the intransitive conform, of the first sense:
to be similar or identical
also : to be in agreement or harmony —used with to or with
The be in be identical is a verb, or at least acts like one enough to be inflected and used as a verb. The technical term is predicate adjective; the be does the predicate part, and the identical does the adjective part.
There are also predicate nouns, like be a dentist. All of these can be treated as single, normally intransitive, verbs of more than one word, like sit down. Since verbs are the prototypical predicate, we don't hafta call them "predicate verbs", but they're in the same category.
Since English can make predicates out of just about anything, there is no reason to have a different special verb that means the same as a particular adjective, unless there's something special about it.
However, there are several ways to say things, especially negative things, so the actual question provides an example of what's wanted, which can be managed nicely.
The question asks us to insert a verb after never:
No there isn't. Structures like "To be X" don't generally have corresponding verbs. There are not (for example) verbs corresponding to "To be green", "To be tall", to be moving", "to be intelligent". English uses the verb "to be" and an appropriate adjective.
If you are looking at an informal way then you can use "twin".
Diamonds never twin!
In the new age lingo, twinning is the word used when two people wear a matching outfit.
To Be (transitive)
I can say "this diamond is the one I saw yesterday" or "this diamond is identical to the one I saw yesterday", with the same meaning. Although there is some ambiguity in the version with identical.
There is "coincide"
coincide INTRANSITIVE VERB
1.1 Correspond in nature; tally.
‘the interests of employers and employees do not always coincide’
Lexico
HERE we find a list of synonyms for this sense:
tally, correspond, agree, accord, concur, match, fit, be in agreement, be consistent, conform, equate, harmonize, be in tune, be compatible, dovetail, correlate, be the same as, parallel
informal square
North American informal jibe
archaic quadrate
Perhaps one of these will be what the OP searches?
‘Diamonds never match.’
‘Diamonds never concur.’
‘Diamonds never jibe.’