I imagine Jane Marple would have owned a sturdy black umbrella that she would have carried with her everywhere.
Her particular umbrella would have been a stick-type, that would be carried and used like a cane or walking stick and made of polished chestnut worn smooth with time and use- no cheap plastic umbrellas here!
The collar of the umbrella I imagine to be silver and quite possibly engraved with her initials. (The collar is the connector between the curved wood handle and the stick portion).
There are many fine old umbrella shops in London where I imagine her umbrella would have been made.