Green’s Dictionary of Slang

1. to lighten the colour of cocaine in order to make it appear more pure; thus chalking, chemically altering the colour of cocaine so it looks white.

1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 136: chalking chemically altering the color of cocaine so it looks white, for those who think this indicates purity.
2001		ONDCP Street Terms 5: Chalking — Chemically altering the color of cocaine so it looks white.

2. to cut cocaine into lines for snorting.

2001

N. Barlay Hooky Gear 182: One the other Orlas chalk me up a line.