The fraudsters targeted their victims, many of whom were elderly and vulnerable, by cold-calling and promising massive returns, the Old Bailey heard.
All of the gems had been bought from a mainstream diamond catalogue before the gang added mark-ups of up to nearly 30 times.
One - a retired police officer - paid £140,000 for stones worth little more than £11,000, the court heard.
Adam Simmons, 27, his father Michael, 53, and brother-in-law Adam Leach, 29, had denied fraud. Michael Simmons' step-son Lee Miller, 32, pleaded guilty to fraudulent trading shortly before the case began.
Adam Simmons was jailed for six years, his father for 30 months and Leach four years. Miller was jailed for 40 months.