His Cutie Mark is a ring of colorful rocks, representing his way to control subterranean stones, surface rocks and earthquakes.
Today, he has the title of lord and he resides in Hoofington. He did something no other Alicorn ever did before (after losing his first wife’s life to Kurayami’s minions, leaving him and his daughter, Mi Amore Cadenza, alive), marry a mortal Pony (in this case, Earth Pony) and have a son, named Blueblood (so Prince Blueblood is half Alicorn, half Earth Pony). No one really knows whether this is a good thing or a bad thing.
Read more about Geo in my seventh FanFiction.net story, Legends of Celestia.
PS: I picture him having the voice of Christian Slater