Thieves have made off with millions of dollars-worth of jewelry and gemstones being transported in a Brink's armoured vehicle in California en route between trade shows, according to the authorities.

The jewelry was loaded onto a truck at an event hosted by the International Gem and Jewelry Show in San Mateo, near San Francisco, on July 10, and was being shipped overnight to another event at the Pasadena Convention Centre.

The robbery took place at a remote truck stop in Lebec in southern California, when the tractor-trailer's armed security detail stepped away from the vehicle. It was discovered in the early hours of 11 July when the guards noticed the lock had been tampered with.

The investigation has not yet determined whether the theft was opportunistic, or the result of a targeted heist with the thieves shadowing the vehicle in transit.

A spokesperson for the International Gem and Jewelry Show said that 18 victims were reporting losses worth $100m from the theft, although a Brink's statement put the value at less than $10m.

The discrepancy is likely to have emerged because jewelry vendors – which are often small businesses – sometimes undervalue goods in transit in order to reduce insurance costs.

"We are working with law enforcement, and we will fully reimburse our customers for the value of their assets that were stolen, in accordance with the terms of our contract," said the security company in a statement.

Freight security firm CargoNet said in a recent report that cargo theft losses in the US and Canada rose to $19 million in the first quarter of 2022, representing a 73 per cent increase over the same period of 2021 despite the number of thefts staying fairly level.