Mainly a terminology thing, but "concrete" wouldn't be used for jewelry. That term refers to a mix that includes large aggregate that's held together with a cement binder. On something like a road or sidewalk, the layer of rocks provides the strength and the cement just holds it in place. On relatively thick art castings, you might be able to use a filler of aggregate the size of say pea gravel or aquarium gravel if you need compressive strength. On small items, especially jewelry size, there would be no need for aggregate, and its presence could even work against you. If you wanted the appearance of the aggregate, you might need to make an over sized lump containing aggregate and then cut it to size with something like a diamond blade.
At the size of things like cement countertops, large bowls, and art sculptures on buildings, what's used is typically a mix of sand and cement, sometimes with strengtheners like embedded fibers. At jewelry size, it's typically sand and cement. The particle size used depends on the item size and surface texture/detail desired. For example, if you're casting tiny beads or something that will have a smooth surface with fine detail, you would need a very fine particle size.
If your objective is something that looks like concrete, with a grainy surface texture, you can use products sold as concrete toppers (the particular product selected wouldn't make much difference for your purpose). These are designed to add a thin layer on top of existing concrete as a repair or leveler. They are generally very strong because a thin layer has to hold up to wear and tear.
If your objective is something that looks more like polished stone, with a very smooth surface and/or fine detail, there are a few products designed for that purpose. One is cement mixtures designed for countertops. Another is a product that's popular among hobbyists for small castings, made by Rapid Set, called Cement All.
Image courtesy Home Depot
It's available at hardware stores, and package sizes include relatively small quantities that will go a long way if you're just using it for jewelry. You can mix it to a soupy consistency and make extremely detailed castings (if the mold is very smooth, the result will look polished and shiny, like polished stone).
The filler is very fine silica, and the binder is CSA cement rather than Portland cement. CSA cement is superior to Portland cement in that it is stronger, doesn't shrink, reaches full strength faster, etc.
You don't need to add anything to these cement mixes for strength, they will be plenty strong for jewelry purposes. There are dyes you can add for a desired color. Concrete is porous and can absorb stains if it's handled a lot. You can seal the surface with a sealant designed for countertops, coat it with resin, or just spray it with a clear finish. Cement can be brittle, so dropping it on a hard surface could chip or crack it. Coating it with resin or a clear finish would provide additional protection against that.