The Blue Diamond battle lines could hardly have been more clearly drawn than by the flying filly Earthquake and the outstanding colt Rubick at Caulfield on Saturday.

The Sydney pair staked their claims for Victoria's premier juvenile race with brilliant wins, both overcoming difficulties and both proving totally dominant.

Earthquake threw down the first challenge in the fillies' division of the Blue Diamond Prelude (1100m), sitting outside the leader Eloping, muddling a stride or two around the turn and then exploding away from the field to score by 2-3/4 lengths in 1m03.71sec.

The performance earned her favouritism for the $1 million Blue Diamond at Caulfield in two weeks, and filled trainer Peter Snowden with confidence.

"She's got natural ability and will take a lot from today's run," Snowden said.

"The benefit from being around here today will be a big plus for her in a fortnight."

The most compelling evidence to support Earthquake's Diamond claim was her drive to the line.

After dropping the bit at the 500m and then rolling off the track around the turn, she took time to get balanced, but when she did she put paid to her opposition within 50 metres.

Rubick, the subject of a pre-race stud deal that makes him one of the most valuable horses in training, looked like getting into trouble when he missed the start in the colts and gelding division and was forced to push hard to retain the inside running.

He then matched strides with the Snowden-trained Mohave to the turn, cruised away entering the straight and held on to score by a length.

"He'll improve, bloody oath he will," said trainer Gerald Ryan.

"There's a lot more he's got to give."

Ryan explained that Rubick's biggest problem - and perhaps his only one - was his appetite.

"You like good doers, but not as good as him," he said.

As popular as Rubick's win was thoroughly expected, it was greeted with great relief by Michael Kirwan the Australian manager of Ireland's Coolmnore Stud which announced on Saturday it had bought a 20 per cent share in the colt.

"It's what you like to see when you've just bought a horse like this," Kirwan said.

Rubick ran the same time, down to one-hundredth of a second, as Earthquake, posing a difficult problem for bookies and punters alike as the assess Diamond chances.

The general consensus had Earthquake favourite as short as $2.50 but with $2.80 available.

Rubick is best-priced at $3.20 with the Gai Waterhouse-trained Nayeli next at $8.