This fascinating form of button was developed in the Netherlands, where 18th century male residents of Zeeland wore it as part of their traditional regional costume. The "Zeeuwse knopen" are convex cannetille filigree buttons made using coiled and twisted gold and silver wires. The golden ones were worn at the neck to close the shirt at the top. The bigger silver ones were trouser buttons to close the long flap trousers of black velvet or wool on the front. Those who could affort wore "gold under the throat and silver on the belly". Boys and poor men wore trouser buttons made of coins.
Today there are almost no people left who wear this traditonal dress. While young people do not wear it anymore, there are only a few older people who wear it occasionally. However, there are still quite some families who own (parts of) this traditional costume and you see them wearing it at summer festivals, feasts and other cultural festivities. Lots of these traditional buttons have been saved and transformed into necklaces, broches, earrings etc., to begin a second worthy life.
For some young Dutch Jewellers it's a new source of inspiration; see the stunning work of Willemijn de Greef (go to "Zeeuwse knopen" at the bottom).
There's even a plant that 's named after this "Zeeuwse Knoop" (Astrantia).