Since this is for an OS X app, it shouldn't be square. For example, review Apple's OS X Icon guidelines,
Additionally, OS X icons need to come in various sizes, although I'm only expecting one base artwork. The 128x128 icon is the main focus. There should be a 32x32 icon, so that it doesn't just look like a blurred version of the 128 icon. Additionally, a 16x16 icon should exist, but only so that it's properly recognizable instead of a blurry 16 pixel square. The user will typically see this in the Finder when browsing their Applications folder. A 512x512 icon and a retina version, 1024x1024, are also required, but extra effort does not need to be made for these to look good; just focus on the design for the 128 pixel icon (you could just make a 512 pixel image so that it looks good at 128).
It's also important to note that while the design should look good at 128 in terms of detail/texture, it would typically appear in the Dock much smaller than that, like 48 or 64. So it shouldn't look too busy or more difficult to recognize at those sizes.
I've attached examples of what OS X icons should look like. Note that they're not based on square backgrounds and instead look more like objects you can interact with. The applications for the pictured icons are Preview, Xcode, VMware Fusion, Transmission, SourceTree, Skype, and Keychain Access.
The Preview and Xcode icons are Apple applications. Note that they look like a document with a tool resting on it. This is the type of thing I was referring to when I mentioned a Slingshot at rest on a surface.
The icon doesn't have to be as detailed or as realistic looking as these; something like the Skype icon could work (though note that the shape is Skype's logo, so I don't want that shape at all). Notice how the Skype icon has a slight drop shadow around it that makes it appear more like an object sitting on your computer screen. Similarly, click the above link and look at the large sample icons.