NEW DELHI (Thomson Reuters Foundation) - Campaigners urged automakers on Thursday to be more transparent in how they source the mineral mica, which adds a shimmer to cars, after Mercedes-Benz boosted efforts to ensure its India supply comes from legal mines free from child labor.

German carmaker Daimler, owner of Mercedes-Benz, announced in June it was increasing inspections and transparency in its sourcing of mica - used to add a shimmer to its cars - to prevent child labor, forced labor and inhuman work conditions.

It audited three mica mines in India, from where it sources the prized mineral, and "discovered and assessed isolated abuses and took corresponding measures" such as dropping one operator from its supply chain.

"The mining of glimmer has repeatedly been connected with child labor in India. The company consistently pursues such indications ... Therefore Mercedes-Benz Cars has decided to make the supply chain transparent," it said in a statement in late June.

Ravi Kant, founder of the New Delhi-based anti-trafficking charity Shakti Vahini, said he hoped the move would push other companies to do the same.

"Once one company, especially a big multinational, decides to take the lead, others should follow. It will be great if others do this because there are slavery footprints in many of these companies," Kant told the Thomson Reuters Foundation.

In August 2016, a Thomson Reuters Foundation investigation found several children in India had died in the depths of illegal mica mines - but that their deaths were covered up.

The discovery that seven children had died in two months alone prompted pledges by multinationals sourcing mica from India to clean up their supply chains, and state authorities vowed to accelerate plans to legalise and regulate the sector.

The investigation, coupled with a study of the industry by rights group Terre des Hommes (TdH), prompted the world's top-selling carmaker Volkswagen to start investigations into its suppliers in India, prompting it to stop buying from some.

TdH called Mercedes-Benz's latest step "a success" in the path to eliminating child labor.

"Terre des Hommes hopes that other automobile companies will follow this first step of Mercedes towards a sustainable mica supply chain," the Geneva-based group said in a statement.

The auto industry is one of the biggest buyers of mica, used in car electronics, paints and coatings, according to TdH.

Mica not only puts the sparkle in make-up and car paint, but is highly flexible, elastic and durable - which makes it vital for several sectors including electronics and increasingly the automobile industry, according to TdH research.

India is one of the world's largest producers of mica, a silver-colored, crystalline mineral that has gained prominence in recent years as an environmentally-friendly material.

Labor rights campaigners estimate that up to 70 percent of India's mica is produced in illegal mines falling into disrepair.