In the past, the social necessity for mourning wear was such that outfitters actually had mourning departments!
The article below was printed in the Australian Town and Country, 29th June 1901 page 43.
Jewellery and buttons were not exempt from the demands of correct mourning.
Even before Queen Victoria wore black Jet jewellery and buttons (very expensive and not very durable), black glass buttons were ‘acceptable’ wear during mourning. A lock of hair of the deceased could be encased under glass for a brooch or (less commonly) a button. Hair could be plaited or woven into bracelets or art works. In the days before photography, this were precious momentos.
Other symbols of mourning that could adorn buttons were the omega symbol (Ω) and angels. Livery buttons could be ordered in a blackened finish, or simply painted black.