A mourning ring is a finger ring worn in memory of someone who has died. It often bears the name and date of death of the person, and possibly an image of them, or a motto. They were usually paid for by the person commemorated, or their heirs, and often specified, along with the list of intended recipients, in wills. Any stone is usually in black, typically Jet

Jet (lignite)

Jet is a geological material and is considered to be a minor gemstone. Jet is not considered a true mineral, but rather a mineraloid as it has an organic origin, being derived from decaying wood under extreme pressure....

 stone. They were popular in Victorian times, and earlier; such as this example of a ring commemorating Jeremy Bentham

Jeremy Bentham

Jeremy Bentham was an English jurist, philosopher, and legal and social reformer. He became a leading theorist in Anglo-American philosophy of law, and a political radical whose ideas influenced the development of welfarism...

. In many cases, there is a lock of hair of the deceased placed under the crystal stone.

Some people whose bequeathing of mourning rings is described in their article

• Cesar Picton
  Cesar Picton
  Cesar Picton was enslaved in Africa at the age of six. He was brought to England by an English army officer who had been in Senegal, and in 1761 was "presented" as a servant to Sir John Phillips, a Baronet living in Norbiton near Kingston in Surrey...
  
  , d. 1836, bequeathing 16 rings
• Sir Anthony Browne
  Anthony Browne (justice)
  Sir Anthony Browne QS was a British justice. He came from a family of lawyers; his father, Sir Wistan Browne, was a barrister of the Middle Temple, and three of his uncles served as benchers there, one of whom was Sir Humphrey Browne, a justice of the Court of Common Pleas...
  
  
• Col. Nicholas Spencer
  Nicholas Spencer
  Col. Nicholas Spencer was a London merchant who emigrated to Westmoreland County, Virginia, where he became a planter and which he represented in the Virginia House of Burgesses...