19 January, 2012 10:12AM AEDT

Olivine olivine all around


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A lump of olivine with some large clear stonesSome cut piece of olivine, otherwise known as peridotMountains of rocks at Mt Shadwell ready to fossick throughMore of the space open to fossickers at the Mt Shadwell quarryThe pointy end of a 'bomb'Alan Wood at the Mt Shadwell quarry ready to crack open another 'bomb'

'Mortlake - Australia's olivine capital'. That's what the signs say as you drive into town, but what is olivine and why is there so much of it around Mortlake?

Alan Wood has been fossicking for olivine for some years now and has built up an impressive amount of knowledge about the mineral and what you can do with it.

As he explains, olivine is actually very common - in fact it's one of the Earth's most common minerals, and exists in many different forms - it's even been found on meteorites and other objects from space.

In the case of Mortlake, the olivine would've formed around 70 - 80 kilometers below the ground and was brought to the surface during volcanic eruptions with the immense amounts of gas shoting up the volcano flinging it far and wide.

Much of the olivine around Mortlake is contained in what are called 'bombs' which look like tear drop shaped lumps of rock. According to Alan, these 'bombs' range in size from something as small as a pigeon's egg to ones as big as a car.

When fossicking for olivine it's merely a matter of finding a 'bomb' and then cracking it open with a hammer.

About 90 per cent of olivine is an olive green colour but sometimes you'll find goldy coloured pieces as well which are more valuable.

Most of the olivine you'll find around Mortlake looks like tightly packed green sugar and will crumble away if you rub it.

While a lump of small green olivine crystals can look quite pretty, the real value is in the gem quality olivine - and at the Mt Shadwell quarry, if you're lucky, you might find a larger piece of olivine which can be cut into a gem stone, or peridot.

Fossicking at Mt Shadwell is easy - you just need to visit the Moyne Shire office in Mortlake first to sign a piece of paper, and then when you arrive at the Mt Shadwell quarry you go to the site office where they'll give you a reflective vest to wear.

On the day we visited there was a large pile of rocks at the quarry - Alan found plenty of bombs to crack open, but none contained a gem stone of any useable size.

It's important to stay out of the way of the people who work in the quarry as well of course, but generally it's not too hard to find a quiet spot and to start exploring - just don't forget your hammer and a pair of gloves!