Because actual granite is expensive
Look for the floral foam section in the craft store. It's the same place every kid get's the foam balls to make their "diorama of the solar system" for the school science fair. Floral foam is usually sold in green and white blocks of various shapes and sizes. It's consistency can range from stiff and crinkly to soft and mushy. Go for the more rigid stuff.
First step is to get it into the rough shape you want by rounding the corners and edges. It's pretty dense stuff and harder to carve then it looks but because it's so porous you can crush and shape it pretty well with your hands. Then I draw in the designs and wording I wanted with a sharpie. Careful as even a sharpie can scrape the foam's surface. Once laid out I then use my trusty Dremel to carve the designs and wording into the foam.
I screwed up the lettering towards the end here so to cover that up I've carved out a ragged chunk and will make it stick out of the ground at an angle.
The material is really part sponge part Styrofoam so it takes a lot of primer to make this stuff actually paintable.
Once it's decently primed we hit it with some spray paint. Starting with black and then highlighting with gray.
Now we apply some texture highlights with acrylic paint and darken the lettering.
The broken tombstone came out better than I expected especially since I botched the lettering and wasn't initially impressed with my paint job. My weeping angel could probably use some more definition but it's not bad for a start.
One of these days if I get enough foam I can turn the back yard into a big styrofoam mosuleam.
A sporting goods store will usually have aluminum tent stakes for under a buck if you need to steak things down. Also if you can find small metal kebob grilling skewers these are just as effective.
The foam is pretty forgiving if it chips. If a small chunk breaks off you can sort of crush it back into place and it'll just look like another natural divot in the "stone".