I live a long way from Finland, and what I know about these hallmarks and the item is gleaned from websites and a book mainly in Finnish.
So first of all the marks:
As I understand the hallmarks they are
1) RM - for Roland Mellin
2) Crown - for Finland
3) 18K - 75% pure gold
4) Boat - town mark for Helsinki
5) R3 - year mark for 1874
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This piece of jewellery is my personal property, and the pictures are my personal IP, should you wish to add them to the 925-1000.com database and website you are welcome to use them as you see fit, I authorise their unrestricted use on 925-1000.com.
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So this is my understanding of a brief history of Roland Mellin jewellers of Helsinki, Finland in the C19th, I will be pleased to be corrected on any information that is incorrect or incomplete.
Swedish born Roland Mellin (1803-1871) became a master goldsmith in Helsinki in 1833, then founded the firm of Roland Mellin, the preeminent Finnish firm of gold and silversmiths in the 19th Century.
His son Otto Roland Mellin (1834 -1904) was among the foundation graduates of the Technical Realschule in Helsinki, where classes commenced in 1858. He continued his studies in St Petersburg and Paris, and succeeded his father in their firm in 1868.
Otto is best known for his Archaeological Revival jewellery based on Nordic artefacts. Among his claims to fame was his position as court jeweller to Finnish born Aurora Karamzina, a long-time member of the inner circle of the Romanovs (the Russian Emperors ruled the Grand Duchy of Finland as an autonomous part of the Russian Empire from 1809 to 1917).
So now here is the piece of jewellery:
This is a brooch 45mm wide and 60mm high including the drop.
It has a removable bale that allows it to convert to necklace, I am not sure whether this is original or a later addition.
The stones are almandine garnets.
I have a book on the Exhibition at the Helsinki Museum of Roland and Otto Roland Mellin, dated 24.3 - 11.8.91
The section on Otto Roland Mellin was written by Ulla Tillander-Godenhielm
On page 38 a brooch dated 1870 is shown with a similar (same?) frame however the insert to the frame is a geometric design.
On page 43 a pencil drawing is shown of a floral bracelet that was Otto Roland Mellin's work for his admission as a master in 1860.
The piece shown above is more like Mellin's master work than his later archaeological revival designs.
My personal conclusion about this piece is that it shows an interest in naturalistic form and may even reflect an early influence from the nascent Arts & Crafts movement, as opposed to the archaeological revival pieces for which Otto Roland Mellin is most famous.