The 10-tonne boulder, from the same quarry that provided the stone for the piers of the Sydney Harbour Bridge, was unveiled on Sunday.

The ceremony, at the garden off Lady Denman Drive, also marked the beginning of international Earth Science Week.

Opened in 2013 as part of the Centenary of Canberra festivities, the rock garden will ultimately house "well over 100" significant rocks and boulders according to director Brad Pillans.

"As I've said on many occasions Canberra has a national botanic gardens, it has a national arboretum, it has a national zoo and aquarium and it has a national library etcetera," Professor Pillans told the 200 strong gathering.

"Now it's appropriate we should work towards having a national rock garden. After all, you don't have to wait for the rocks to grow; when they come here they're fully grown. You don't have to water them, you don't have to feed them and they don't bite. So I think there is a lot going for the concept of a rock garden."

Professor Pillans made a spirited defence of Canberra's fickle climate on the grounds that 470 million years ago the site of the rock garden had been a tropical sea complete with coral but totally free of sharks as they had not yet evolved.

"Had we been standing here 430 million years ago we would have been floating in a warm, tropical ocean. The next time someone tells you how bad the Canberra winters are you could just casually drop into the conversation 'if only you had been here in the Silurian period 430 million years ago; how much better things would be'."

The latest addition to the inventory, while 50 million years later in time, was still a very special chunk of rock.

Donated by the Moruya Quarry, the block of tonalite arrived at the site in May where it has been shaped and polished and fitted with an instructive plaque.

Moruya tonalite has been a highly regarded architectural and building granite for more than a century. In addition to its important role holding up the Sydney Harbour Bridge it was also used in the construction of the Sydney GPO and for the base of Hyde Park's Captain Cook statue.

A 20-tonne block serves as the altar stone at Sydney's Martin Place cenotaph.