The attackers from the al-Shabab group shot the non-Muslims dead after separating them from Muslims, residents said.
There is growing concern in Kenya about security in regions bordering Somalia after a spate of attacks.
President Uhuru Kenyatta, who summoned top security officials, is to address the nation on TV later on Tuesday.
The attack on the quarry workers took place earlier in the day. Witnesses said the victims were caught after midnight, while sleeping in their tents at the quarry in Kormey, 15km (nine miles) from Mandera town.
A driver who visited the scene of the attack, Ali Sheikh Yusuf, told the BBC most of the victims appeared to have been lined up, and shot in the head, at close range.
He said four were beheaded inside their tents, while three appeared to have escaped to Mandera town.