The pyramid is the plot monument for what was the Bethel family plot. It was designed by J.A. Bohnet, one of the directors who had supported Rutherford.
Plot monuments are distinct from headstones in that they mark whole groups of graves. To that end, they're often very large statues or obelisks. Of course, the really, really wealthy built full blown mausoleums. Neither of these, (Mausoleums and plot monuments) are as common today as they used to be, but you can usually find both in an old cemetary.