The fissure cuts through Greek figures representing Peace, Victory and Valor, said Arlington Superintendent John Metzler Jr.
Miners are searching for matching marble, returning to the old Colorado Yule Quarry site in the central part of the state where the original marble was found.
It should take a year to complete the work after the mining operation is finished, Kerry Sullivan, a spokeswoman for the cemetery, said Monday.
The crack was first discovered in the 1940s during the Truman administration, repaired in 1989, but has now become more visible again.
The blemish may be unsightly, but it does not diminish the emotional experience for visitors, said Sgt. Paul Basso, of the 3rd U.S. Infantry "Old Guard" Regiment that provides security at the tomb.
Basso said he's never had anyone ask specifically about the crack.
President Bush laid a wreath at the tomb Monday before addressing the annual Memorial Day observance at the cemetery.
The rectangular memorial, sitting on a Virginia hill overlooking Washington, solemnly tells visitors: "Here Rests in Honored Glory An American Soldier Known But to God."
The tomb has been at the site since 1921, more than a decade longer than the memorial, when an unidentified soldier from World War I was buried there.
Sculptor Thomas Hudson Jones carved the memorial with wreaths, the three Greek figures and the inscription.
While unknown soldiers from World War II and Korea also are buried at the site, remains from the Vietnam War were disinterred in 1998 after DNA testing identified them as Air Force 1st Lt. Michael Blassie.
Members of the Old Guard regiment provide security at the tomb 24 hours a day, their dress uniforms, polished buttons and changing-of-the-guard ceremony watched by 4 million annual cemetery visitors.
Once the new marble is located, a master carver will have to copy the original design by architect Lorimer Rich.
"Right now we're concentrating on finding the best piece of marble," Sullivan said.