Whilst today 90% of memorials are made from granite, alternative materials are Sandstone Slate and Marble. Traditional or modern shapes. Ornamentation can be additional detailing added to the memorial such as bronze statues or ceramic portraits. Alternatively craftsmen carve or etch designs into the stone.
The most important and difficult decision is what words you wish to put on the stone. Remember that there is no "correct" way to do this. Words are a very personal thing and, as long as they don't stray outside the bounds of what the burial authority will allow, you should say what you want to say. The form of the words need to compliment the shape and structure of the memorial.