Corundum (aluminum oxide) defines hardness 9 — only diamond among common references is harder, so corundum scratches everything else in the kit and leaves no streak on a porcelain plate. Red gem corundum is ruby; every other color is sapphire. Look for six-sided barrel or bipyramidal crystals with basal parting in metamorphic rocks and gravels.
Anchoring the scale
At Mohs 9, corundum is the practical ceiling of field testing: it scratches every common reference and yields only to diamond. That makes a small synthetic corundum point the most useful upgrade a serious kit can take — the "Beyond 8" brief covers the options and the honest limits. The bottom of the scale belongs to talc, which puts up no resistance at all.