"Diamond watch fob" could refer to several things:
* A specific historical object: Perhaps you're asking about a particular diamond watch fob that was owned by someone famous or is part of a museum collection. If so, you'll need to provide more details about the object itself.
* A general trend in jewelry: Maybe you're wondering about the popularity of diamond watch fobs in the past and why they are less common today. Fobs were once more popular as a way to attach pocket watches to clothing, but with the decline of pocket watches, they've also become less common.
* A fictional item: It's possible you're asking about a diamond watch fob featured in a book, movie, or TV show. In this case, you'll need to specify the work of fiction.
To help me answer your question, please provide more information about what you are asking about.
For example, you could tell me:
* Where did you see the phrase "diamond watch fob" used?
* Do you have any more details about the object, such as a specific time period or owner?
* What is it you are specifically looking for?
The more information you give, the better I can understand your question and provide a helpful answer.