Help required with marks please

[Daisy Banks]

Hi, I'm new here and have looked through the site but can't find the info I'm looking for. I have a paste brooch which I bought from Bermondsey market 20 years ago. It is shaped like a tiara and has 900 on the back plus a tiny mark which is a diamond in a rectangle. Could it be French? Can anyone advise on what this is? Thanks in advance.