The attackers from the Somalia-based Islamist militant group separated Muslims from non-Muslims and shot the Christians dead, residents said.
Earlier, one person was killed in a bar popular with non-Muslims in a neighboring district.
Al-Shabab killed 28 people in an attack on a bus targeting non-Muslims in the same area last week.
The attack on the quarry workers took place early on December 2. Witnesses said the victims were caught after midnight, while sleeping in their tents at the quarry.
The attack took place in Kormey, 9 miles from Mandera town.
Al-Shabab said it carried out the attack.
Kenya’s Red Cross said on its Twitter feed that security personnel and one of its own teams were on the ground at the site of the attack.
In the attack on a bar in the town of Wajir earlier on Monday night, gunmen reportedly opened fire and hurled grenades, killing one and injuring 12.
Mandera County borders both Somalia and Ethiopia, and it is dominated by Somalis, who are largely Muslims.
Many of the quarry workers killed are reported to have come from the south of the country where Christians predominate.
Al-Shabab has stepped up its campaign in Kenya since 2011, when Kenya sent troops across the border to help battle the militants.
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