Talc (a hydrated magnesium silicate) defines hardness 1 — everything scratches it, including a fingernail with ease. The soapy, greasy feel of its foliated masses is the classic field cue, and massive talc is the soapstone of carvers and countertops. Found in low-grade metamorphic rocks, often with serpentine.
Anchoring the scale
Talc is Mohs 1 by definition, which makes it the calibration point for every scratch kit: if your "talc" resists a fingernail, your reference is mislabeled, not the scale. Its other unmistakable cue is feel — soapy and almost slippery, a texture no harder mineral shares. The top of the scale is held down by corundum, and the library covers the whole run between.