Short answers, pulled from the story.
Thomas earned the nickname at the Battle of Chickamauga on the 19th of September, 1863, when he rallied broken Union units on Horseshoe Ridge while the rest of the Union line collapsed. Field officer James Garfield, a future president, reported to General Rosecrans that Thomas was "standing like a rock," and the nickname spread from that account.
Thomas attacked Hood's Confederate army on the 15th of December, 1864, and destroyed it in two days of fighting. Congress passed formal Thanks to Thomas and his soldiers for their "skill and dauntless courage" in defeating and driving Hood's army from Tennessee. The victory earned Thomas the additional nickname "The Sledge of Nashville."
Thomas struggled with the decision but ultimately chose loyalty to the United States over his home state. His Northern-born wife, Frances Lucretia Kellogg, is believed to have helped influence his decision. He also turned down Virginia Governor John Letcher's offer to become chief of ordnance for the Virginia Provisional Army, which clarified his allegiance.
Thomas's Virginia family turned his portrait against the wall, destroyed his letters, and never spoke to him again. After the war, when Thomas sent money to his sisters during hard economic times in the South, they refused it and declared they had no brother. None of his blood relatives attended his funeral in 1870.
Thomas destroyed his private papers, refused to write memoirs, and died in 1870, only five years after the war ended. Grant's own memoirs minimized Thomas's contributions, and Sherman's repeated description of Thomas as "slow" shaped later perceptions. Most of Thomas's campaigns took place in the Western theater, which received less press coverage than the Eastern theater.
Thomas is buried in his wife's family plot at Oakwood Cemetery in Troy, New York. His gravestone, sculpted by Robert E. Launitz, is a white marble sarcophagus topped by a bald eagle. An equestrian statue stands at Thomas Circle in Washington, D.C., commissioned in 1879 by veterans of the Army of the Cumberland.