Formal:
* That extraordinary timepiece is a valuable historical artifact.
* That unique clock is a prized heirloom of bygone times.
* That distinctive timepiece is a rare and valuable piece of antiquity.
Informal:
* That weird clock is a real treasure from the past.
* That old clock is a super rare find.
* That strange clock is worth a lot of money.
More descriptive:
* The intricately carved clock, with its unusual design, is a priceless antique.
* The clock, with its unique mechanism and intricate details, is a rare and valuable piece of history.
The best synonym for you will depend on the context and tone you want to convey.