Scottish Thistle Brooch

I love brooches, and often wear them, and I have noticed there are some very knowledgeable brooch collectors on here, so I thought I would pick your brains about this one. No marks and it was pretty black when I bought it, with a nice large blue glass stone in the middle with smaller stones round the edge, and with a design of thistles and leaves round it. I have cleaned it up a bit with silver polish, and it looks a lot brighter now. It's 4.5 cms across.