Many bishops wear a cross or a ring with a purple stone in it. Ever wonder why?
The book of Revelation gives a symbolic description of Heaven, saying it is like a city with 12 foundations, each of which was associated with an Apostle: "And the wall of the city had twelve foundations, and on them were the twelve names of the twelve apostles of the Lamb." (Rev 21:14)
It goes on to say that "The foundations of the wall of the city were adorned with every kind of jewel." (Rev 21: 19a) The twelfth foundation was adorned with amethyst (Rev 21: 20, end).
Now who was the 12th Apostle? It was Judas Iscariot, who after falling away was replaced by Saint Matthias (whose feast day is today). Saint Matthias, therefore, was the very first person to be a successor to an Apostle.
This custom of having successors to the Apostles continues to this day, only now we call them something else: bishops. And since Saint Matthias, the first successor to an Apostle, is associated with the 12th foundation of the city (adorned with amethyst), other bishops often, by custom, also wear a purple stone to show that they too are successors of the Apostles.
And that's why I wear a pectoral cross with a purple stone in it. Saint Matthias, pray for me, and for all my brother bishops, as we try and follow in your footsteps as successors of the Apostles!