Check it out, it's a rock! Actually it's a super cool glacial Antarctic rock, brought from Mawson station and put here to commemorate the centenary of the Victorian Royal Society. That was back in 1959. I worked across the road from here for years, and I didn't come over to read the plaque until much later when I was visiting the city with the kids.
It's on the corner of Exhibition and LaTrobe streets, so go and see the Antarctic rock!