Hi Zeide,
no, I dont have any particular grounds for thinking Scottish......only my ignorance about pearl types!
That it may come from Broome makes more sense. I know that 15 ct gold was discontinued in 1931, and I would think that a British item would have all the hall-marks, so it is more likely Australian made. Unusually, the pin itself is gold.....most being gold plated, separate from the brooch.
Pearling for the shells was big business in the later part of the 19th century, first part of the 20th......used for knife handles, buttons, inlays and such. I guess its possible that the odd pearl was found and put into jewellery......its a romantic thought anyway.
Whether its a natural or cultured pearl, I dont know......and I was wrong on the size too.....after checking it is a hairline under 11 mm.