The commentaries see this as a reference to the rock on which the Jewish people is founded – their ancestors.
Explaining the link between flintstone and ancestors, Rabbi Joseph Isaac Schneersohn remarks that a flintstone has the property of producing sparks when struck by another stone.
Further, such a rock can remain in water for centuries and still be able to produce fire: dry it out, strike it and a spark will result.
Rabbi Schneersohn says the same thing applies to the Jewish people. Even if submerged for years by other, sometimes unhealthy, influences, a Jew has a pintele Yid, a spark of Jewishness that can never be completely extinguished. Jews and their ancestors are never spiritually severed from one another.
To which we might add that this applies not only back across the centuries, but in the contemporary situation too.
An American rabbi, Morris Adler, said that a Jew is a person who never meets another Jew for the first time. A spiritual and cultural spark of kinship is truck even when you encounter a fellow Jew whom you think you have never met before.
Meet a relative for the first time and you are often struck by a resemblance; meet a fellow Jew and you recognise a common patrimony.