It’s lovely, it’s value is a bit more than its components because it has its lovely fitted box and it’s Scottish.
I presume the pin is hallmarked, you might be able to date its manufacture from the date code (it’s a letter).
I don’t buy brooches so I don’t follow their prices but I would think $200 - $300.
Edward Diamond Merchants was the name of the jeweler then on Buchanan Street in Glasgow. See, e.g., their case for this antique pin:
Victorian Diamond and Seed Pearl Yellow Gold Brooch For Sale at 1stDibs
For Sale on 1stDibs - A fine and impressive 0.05 carat diamond, 15k yellow gold brooch; part of our diverse Victorian jewelry and estate jewelry collections. This fine and impressivewww.1stdibs.com
I think "To His Majesty" is shorthand for "By Appointment to His Majesty," That, plus the seal, would indicate IIRC royal approval of the merchandise purchased /commissioned from the jeweler.
Unfortunately, the Royal Warrant Holders Association does not maintain an online roster of Warrant holders in the past, just current ones:
Royal Warrant Holders Association | Public home page
www.royalwarrant.org