A femme lesbian or feminine woman who does not allow or enjoy sexual penetration; she may not even allow any touching of her genitalia whatsoever, even clitoral stimulation. She generally prefers to "top," or be the more active partner when having sex.
"Stone" can also be used to refer to butches who prefer to top and not be touched sexually themselves.
That girl is a total stone femme; she will strap it on and give it to you good, but don't even think about trying to touch her. She is strictly a top.