I propose vacuum forming the shape that we want, adding pins in the right location for the speaker holes, using epoxy resin (or maybe AB foam) to coat the inside of the mold, and finishing the outside of the mold with that stone-like spray paint (but use an epoxy coating first if it's AB foam). It's probably possible to calculate the displacement of the rock if we want to tune the speaker or add porting. Perhaps make them active speakers by building a small amp? Perhaps an ipod / ipad dock?

Count me in for the discussion and probably the build - I would love to have a pair of these as well. I'm available pretty much anytime starting Wednesday.

I don't need any outdoor speakers, but I can help with or lead this build if you'd like. FG would be the easiest method and be more portable, cement would give the best sound but would be heavy. Rocks are fun to sculpt, difficult to make a rock "wrong".

Molding resin would be the product to use if you wanted to make multiples of the same shape/size, because it reproduces fine detail very accurately, is weatherproof and can be tinted so any chips would not be noticable. The downside is that it requires an accurate model real or sculpted, an expensive to make complex rubber mold of both the outside and inside (the resin is a liquid so it would make a solid rock if you just molded the outside surface), and the resin is expensive for a project this large. This all would make it more of a factory process than a one of a kind.
Realistic rock is not difficult to model if you are observant and really look at example natural rocks. I'd suggest starting to collect photos of rocks that you like for their shape, texture and/or colors (not necessarily all in one rock). If you can find some fist sized samples, collect them as well. Rock shops will have a number of different type rocks to look at/photograph.
Different rocks will have different characteristics: water worn rocks look totally different from field rock, igneous rock looks different from sedimentary rock. The most natural looking "fake" rocks are going to resemble the native rock found around them. "Feature" rocks are transplanted rock not native to the area used as a focal point in a landscape or arrangement or to create a "scene".
Try and focus on what type rock you find most attractive and then on what characteristics are in common throughout.

I would build a styrofoam beadboard "buck" for the inside and support. Cover it with a polyethylene bag (cleaner's bag would do) to make removal of the foam easier. Then lay up a combination of FG mat and cloth to a suitable thickness. I would then use a layer of Bondo over the FG and various tools to sculpt the details and texture. Don't like what it looks like? Grind off the Bondo and redo, or even saw off a portion of the FG and rebuild. Once it is to your liking, (easily) pull out the foam, give it a coat of primer and paint. Making a mold from a real rock would be time consuming and expensive for a one time use, especially if you wanted to rotocast it. Then there is the problem of where do you get a rotocast machine? The cost of your two rocks just went into multi $K each! Not trying to shoot down your ideas, but there's a big difference between imagining how to do something the "if I had unlimited funds" way (you could just go out and buy them, or hire me to make them! :) ) and what's practical and cost effective way to produce a real product. I think John would actually like to end up with an actual pair of rocks and probably would prefer they did not look identical to each other.

Biggest reason people say they can't draw or sculpt is they don't really observe. When they draw a house they don't look at what real houses looks like, they draw the sterotype they were taught pre-elementary school, a box with an inverted V on top then add a couple more boxes to represent a door and windows, put a chimney on the roof and finish it with a curlicue of "smoke" coming out of the chimney. The rocks at the miniature golf course for example, the people didn't try to look at real rocks and reproduce their characteristics, they built their (simplified) vision of rocks from their mind's eye. Take a look at the sculpted "rocks" at Disney World and you can't tell them from the real thing.

I almost deleted the post above as spam, but hey, he does admit to the shameless promotion and it was obviously written by a human who has read our forum and has one possible solution to the question at hand.

Be aware however, that he is in Huntington Beach, CA, so you probably won't "stop by to take a look", but you can see a lot of pictures on his website.

Scott

Wow, thanks everybody! Lots of food for thought here! The creation of the rocks is almost completely within the Maker philosophy. It's an opportunity to learn a new skill or two and probably a few new curse words!

One of the reasons I downplayed the idea of making a rubber mold is that I have made rubber molds, and therefore I could teach that process as well. It's just that I know how much work and expense it is to do. IMHO unless you are making something small that you also want to produce in multiples it's far from cost effective.
Note the lab fee for the above class: 20.00 to make a mold for an object the size of a quarter.

Jon,
lots of different approaches here for you to choose from.
Now to decide which is the most feasable, within your budget, and interesting to you for your purpose.
If you would like to discuss the pros and cons of my suggestions and meeting your needs further I'll be in the wood shop/shopbot area all Tues afternoon and evening working on the kitchen cabinets.