The "quality" of quartz varies a lot.
The stuff around me is soft enough to almost crumble in your hands. So it does not work well or at all to strike sparks with a flint striker. But some early French explorers up in Canada found outcroppings of quartz hard enough that they mistook them for diamonds. Only after they sent back news and samples of their great discovery was it found out that they had just found HARD quartz - not diamonds. It turned into something of a slang term/phrase - to call something of poor quality "as good as a New France Diamond"! (or "as good as a Canadian diamond" - but there actually are true diamonds in Canada. Just not what those guys had found.)
What you are trying to do is use a hard/sharp edge on your stone to chip/dig out little bits of the steel from your flint striker. The energy you put into chipping/digging out those little bits of hardened steel heats them up enough that the carbon in the steel burns. That's the sparks you see.
Flint just works better. You can get a sharper edge from flint, and it is stronger and last longer in use than with other types of stone. But you can still get sparks using other types of stone. The sharp edges just tend to crumble faster in use. You can use granite, slate, chirt/chert, agate, jade, marble, fossilized wood/bone, and most any other stone. Flint just works better - with a thinner and harder sharp edge.
Hope this helps.
Mikey - yee ol' grumpy blacksmith out in the Hinterlands
Pure clear crystal from Madagascar Ogri - not Welsh Quartzite
