The incident happened on Monday last week at the Luohu crossing, which links to the city’s Lo Wu immigration control point.
A Hong Kong government source said diamonds were smuggled into Shenzhen to avoid import restrictions and mainland taxes that amounted to 20 to 30 per cent of their value.
“We believe the diamonds were being taken to the mainland’s processing factories to make jewellery before being smuggled back to Hong Kong,” the source said.
He said the Year of the Rooster was traditionally deemed a good time to get married so there was high demand for jewellery.
He added that carrying diamonds in or out of Hong Kong via the city’s control points was not a breach of Hong Kong laws.