I have only ever found one occasion on which Princess Louise, Duchess of Argyll was reported to have worn a pearl and diamond tiara. There may be many more -- but I haven't found them.
The occasion was the first State Opening of Parliament in the reign of Edward VII in February 1901.
Unfortunately I have never found an image clearly identifying Princess Louise.
The best I can find are sketches of the scene in the House of Lords.
These two give the best idea of the tiaras worn -- but I don't know if they are an accurate rendition.
The two images came from the same paper but were on different pages.
I attempted to join the two images to make it easier to see the ladies I suspect are Princesses. Unfortunately I had enlarged the second image more than the first!!
Below is another, similar sketch.