Quartzite is a term sometimes used for a tightly cemented sandstone, where the cement is largely SiO2 [silica]. It is generally, however, a metamorphic rock--a thermally and pressure altered quartz sandstone or chert [recrystallized]. Orthoquartzite is a term generally used for a very pure quartz sandstone, with or without much cement. It is a sedimentary rock. The important thing is that both contain [usually] sand grains or coarse crystals and may not have as regular or fine an edge as "flint" [chert]. I would think that a typical "quartzite" would be an agressive cutter of steel--giving good sparks but eating away the frizzen face more rapidly than flint. In some meta-quartzites the rock can break across grains [which are overgrown with SiO2 cements] and give a more regular edge and close to conchoidal fracture. Each specific rock has to be judged on its own merits. Quartzites are dominatly SiO2 with a H = 7, so will cut steel. Chert/flint has been used a long time for good reasons and substitutes IMHO are best relagated to "emergency" usage, rather than longterm regular usage.
