Here are the examples of three subject verb disagreement in this text:
1. "the fate of the diamond... were unknown" - it should be changed to was unknown
2. "neither time nor conflict have dimmed" - it should be changed to has dimmed
3. "the twin eighty-year-old brothers suggests" - it should be changed to suggest
Subject and verb have to agree when it comes to the number - if the subject is singular, the verb has to be singular as well. If the subject is plural, the verb has to be plural.