On Tuesday night, connections of impressive debut winner Burnstone knocked back a $1.8 million overseas offer for the colt.

Less than eight hours later, Burnstone was dead following an apparent heart attack during trackwork at Cranbourne.

In addition to the offer from an overseas client, trainer Robbie Griffiths said there had also been interest from a number of studs.

"It's an emotional rollercoaster racing," Griffiths said.

"We declined an offer at 10 o'clock last night of $1.8 million as the owners didn't want to sell the dream, but were open to selling a quarter share.

"Our next discussion was going to be increasing his insurance from $40,000 after we'd just knocked back the 1.8."

Just 15 minutes before Burnstone collapsed, Griffiths stood looking on in awe at the horse's condition.

He said the colt was preparing for the Blue Diamond Prelude at Caulfield on February 14 and had worked at Cranbourne on Tuesday morning.

"He did a bit of evens on Tuesday morning and at five o'clock this morning I would have told you he was 110 per cent and 15 minutes later he's had a (suspected) heart attack," Griffiths said.

"He's gone off to Werribee (equine centre) for an autopsy but I don't know how long it will take them before they get the results back."

Griffiths described the horse's death as a tragedy.

He said if Burnstone had been injured and unable to race again it would have been devastating but at least he would have had a future at stud.

"He's by a champion sire that was a Blue Diamond winner out of an Encosta de Lago Group One performer whose granddam was Merlene, a Slipper winner," Griffiths said.

"He would have had 150 mares queuing up at $10,000 a pop."

Griffiths will now rely on Deja Blue as his Blue Diamond prospect with the filly making her debut at Sandown on Saturday in the Inglis Premier over 1000m.

"It would be ironic if she won the Diamond," Griffiths said.

"He (Burstone) was potentially the best horse I had trained.

"Some horses have the talent but not the pedigree to take them the whole way.

"He ticked all the boxes."