I have a number of rings that are very small in size. Some may just be the size they are because the original owner had very small fingers but some, I consider, were designed to be worn as pinkie rings. The pinkie finger is the smallest finger on the hand and the name derives from the 19th century and the Dutch ‘pinkje’ which is the diminutive of ‘pink’ meaning little finger. The word ‘pinkie’ was first used in a Scottish Dictionary in 1808.
Pliny the Elder talked about wearing rings on the little finger:
Some people put all their rings on their little finger only, while others wear only one ring even on that finger, and use it to seal up their signet ring, which is kept stored away as a rarity not deserving the insult of common use, and is brought out from its cabinet as from a sanctuary; thus even wearing a single ring on the little finger may advertise the possession of a costlier piece of apparatus put away in store.
But it was not just Roman men who wore pinkie rings. McCarthy refers (at p66) to a list of what a Roman lady had in her jewel box which included two diamond rings for the little finger.
Males have worn signet rings on their little fingers for centuries. You can look at paintings from the Rennaissance in particular to find examples. Not all rings were signet rings though. Henry VIII wore a lot of rings, including gem set rings, on his little finger https://www.npg.org.uk/collections/search/portrait/mw03086/King-Henry-VIII?LinkID=mp02145&role=sit&rNo=5).
There are also historical paintings of females wearing rings of different styles. In a portrait in the Royal Collection of a woman who may be Katherine Howard (see https://www.rct.uk/collection/422293/portrait-of-a-lady-perhaps-katherine-howard-1520-1542), she is wearing a thin band but in the Hans Holbein painting of a young Queen Elizabeth, she is wearing a ring set with a gemstone (https://www.hans-holbein.org/Portrait-Of-Elizabeth-When-A-Princess-Three-Quarter-Length-In-A-Red-Jewelled-Dress-The-Bible-In-Her-Hands.html). Louise of Mecklenburg-Strelitz, Queen of Prussia from 1797 to 1810, wore her betrothal ring on her little finger (McCarthy, p 158).
In the 19th, 20th and 21st centuries, the males of the British Royal family wore rings on their little fingers. There are paintings of Prince Leopold, Duke of Albany, the youngest son of Queen Victoria, wearing rings (for instance, https://www.rct.uk/collection/405896/prince-leopold-1853-84-duke-of-albany and Edward VII (https://www.npg.org.uk/collections/search/portrait/mw02039/King-Edward-VII), and pictures of Prince Philip wearing both his wedding ring and his signet ring on that finger (https://royaltyspeaking.tumblr.com/post/24975382978/xlilibetandphilip-prince-philip-aged-32-in). King Charles III has always worn his signet ring on his little finger.
‘Rings through the Ages’, James R McCarthy, 1945, https://archive.org/details/ringsthroughages00mcca/page/n7/mode/2up?q=little+finger
Pliny’s, Natural History, Book XXXIII, translated at https://www.loebclassics.com/view/pliny_elder-natural_history/1938/pb_LCL394.21.xml
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