Looking at garden photos taken in the 19th century, I noticed that several of these gardens had decorative concrete spheres. I decided to try to make some and use them as in my own garden. Here is how I proceeded.
My problem was how to get a spherical mold. I eventually found a solution. I bought some glass globes that are put over light bulbs. I found mine in a used building materials store where they came in two sizes and were $1 a piece. They no doubt can be found in regular stores.
I then made a batch of concrete, filled up the globes and then let them sit for several days to allow time for the concrete to cure. The concrete should be made exclusively with sand or very small aggregate, otherwise the result is not very smooth. It should also be rather wet.
When the concrete has thoroughly set, I take a hammer and very gently break off the glass globe (they cannot be reused). Naturally, you need to wear protective gloves and glasses, and it is easier to first put the globe in a cardboard box in order not to spread around the pieces of glass. If you give gentle taps with the hammer, the glass breaks off without flying all over the place and without damaging the concrete globe.
I have used globes on each side of paths, as they did in some of the old gardens. Some I have used singly to create a focal point as in the first picture in this post. Over the years, lichen starts growing on them and they acquire a patina. You can speed up the process by spreading yogurt on them. The following one was made about 6 years ago and is starting to age nicely.