Where does “*kalk” come from?

*kalk (Proto-Germanic) comes from Latin calx, from Ancient Greek χάλιξ — small stone, pebble; gravel, rubble.

Ancestry of “*kalk”, step by step

Step	Language	Word	Meaning
1	Latin	calx	limestone; chalk; the finish line
2	Ancient Greek	χάλιξ	small stone, pebble; gravel, rubble