Unfortunately what I think is the best one there, you would need a car to get there and it is outside the city. What are your limits to 'in the city'? There is one worth visiting in Springvale South called Collectors Corner, that would be able to be visited by train to Springvale and bus from there. It also has collectable plants, bonsai, orchids, cacti, etc and a nice cafe. It's run by the same people that have my first choice so there is a good range of higher end rocks and fossils. Other than jewellery suppliers in the city itself, not sure of any others.