You want to use cement, not concrete. http://www.ganoksin.com/gnkurl/ep8114
The site referenced above is basically correct on the difference
between cement and concrete. But I think that concrete, not cement,
is what the person who asked this question would want to use.
Portland cement is just the binding agent for whatever aggregate is
used to make concrete; it has little strength by itself. Mixed with
gravel, it makes a hard building material.
Mixed with sand and clay, it makes a good mortar. Mixed with marble
chips and ground off, it becomes a terrazzo floor.
A similar approach, using 2 parts or more of crushed turquoise,
lapis, chrysocolla or another relatively soft stone to one of cement
would probably make a good “jeweler’s concrete”. I’d leave out the
sand, though, which is too hard; it will grind off more smoothly
without it. Some acrylic admix, mixed into the water used to hydrate
the mixture, would improve its adhesion and durability.
Andrew Werby