Four gem dealers who duped investors into paying hundreds of thousands of pounds for diamonds at hugely inflated prices have been jailed.

The fraudsters targeted their victims, many of whom were elderly and vulnerable, by cold-calling and promising massive returns for their investments, the Old Bailey heard.

All the gems had been purchased out of a mainstream diamond catalogue before the gang added mark-ups of up to nearly 30 times the price they paid in some cases.

One customer – a retired police officer – used up the last of his savings buying gems for £140,000, but the stones were worth little more than £11,000, the court heard.

Adam Simmons, 27, his father Michael, 53, and brother-in-law Adam Leach, 29, denied fraud but were convicted following a trial at the Old Bailey.

Michael Simmons’ stepson Lee Miller, 32, pleaded guilty to fraudulent trading shortly before the case began.

Sentencing the gang, Recorder Michael Wood QC said the men had carried out a “carefully planned and executed fraud” over a number of months in which they pocketed in excess of £900,000.

Cocaine Adam Simmons, who was described as the “mastermind” behind the operation at the No1 Gems firm, based in Hove, received £280,000 from the scam which he spent on “drinking, taking cocaine and indulging himself”, the recorder said.

Simmons, of Booth Street, Old Swan, Liverpool; his father Michael, of Lion Hill, Stone Cross, Pevensey; and Leach, of Hambleton Close, Eastbourne, all denied conspiracy to defraud, money laundering and two counts of fraudulent trading.

Adam Simmons was jailed for six years, his father Michael for two-and-a-half years, and Leach was given a four-year prison term.

Miller, of Sackville Road, Bexhill, who admitted a single count of fraudulent trading, was jailed for three years and four months.

Addressing Adam Simmons, Mr Wood said: “I take the view you are by far the most seriously involved in this case.

“You are responsible directly for your father’s downfall...and effectively destroying your family.”

The judge added that Michael Simmons had taken part in the fraud in a “misguided sense of loyalty” to his son, who had a number of previous convictions.

The men were disqualified from holding company directorships, and court proceedings were ordered |to determine whether any cash can be recovered.