But things took a decidedly unfestive turn for three Melbourne jewel merchants on St Patrick’s Day 1978.

Each man was shot in the head execution-style on the eight level of the Manchester Unity Building, one of the city’s grandest skyscrapers.

Police believed the killer may have posed as a buyer intent on purchasing eight cut diamonds valued at $30,000, which they fled with after the brutal killings.

But early leads failed to produce solid evidence and prime suspect Alex Tsakmakis hit the sack (or to be precise, a sack hit him) without ever facing charges.

Diamonds are said to last forever and so too will the mystery of the Manchester Unity murders.

Was this just a stick-up or could it have been an insurance scam gone wrong?

The time: Between 2pm and 5pm on March 17, 1978

The crime: Three jewellery merchants, Paul Pace, Keith Hyman, both 32, and Robert Wartman, 33, shot dead.

The scene then: Paul Pace’s office, in room 804 in the Manchester Unity Building, at the corner of Swanston and Collins streets.

The scene now: The heritage-listed, neo-gothic skyscraper is one of Melbourne’s best-loved buildings. Today it is home to a number of dental clinics.

Aftermath: The killings remain unsolved but prime suspect Alex Tsakmakis was later sentenced to life for a separate murder and armed robbery.

In 1988, a fellow Pentridge Prison inmate killed Tsakmakis by hitting him in the head with a pillowcase filled with gym weights.

Russell Street bomber Craig Minogue was convicted of Tsakmakis’ murder.