Dear bp57,
Great question.
The pet rock - as sold in the 1970's - was one of those ideas which would have been hard to protect with IP. But it probably didn't matter, the business model was not dependent on IP: the key element in the business plan was timing.
Fads die quickly and after the initial few months surge there usually isn't much profit left. By being first out with the product, Mr. Dahl didn't need IP. All his profit was on the front end and so his period of "sustainable differentiation" was long enough. Once the copycats started cutting into his action, the market was already greatly diminished. He may have wasted his time getting patents to cover his idea and even more money trying to defend his share of a shrinking market.
On the other hand, say the pet rock had staying power... like Barbie (the original pet rock.) Then it would be necessary to design-in features which could be protected with IP. In addition to choosing a catchy name for a trademark, maybe he could have designed in features into the rock which made it eligible for design patent protection, or even copyright protection. Perhaps patent a unique box in which it is sold, or displayed.
While it was not possible in 1975, it might now be possible to patent the business model - it certainly was not obvious that selling rocks would have generated such commercial success.
My guess is that the Pet Rock was never destined to live for more than a short period of time and IP protection would not have made too much difference in the profits Dahl was able to extract.
Your specific idea may be entirely different. If so, then spending a few hours with a patent expert to discuss the details of your circumstances might be money well spent.
Regards,
Eric Stasik