a " 'white rock substance,' " distinguished the substance from powder cocaine, and agreed that crack was " 'vernacular slang for cocaine base’ ”); United States v. Lawrence, 471 F.3d 135, 139 (D.C.Cir.2006) (affirming a conviction where police witnesses testified that the substance was a "large white rock substance” and that the sale “followed conventional practices for the sale of crack cocaine," and where an undercover officer testified that the defendant gave him the substance in response to his request to buy crack). 6 . , The Johnson court cited the opinion in United States v. Eli, which noted a DEA chemist’s testimony that the concentration of cocaine base in crack cocaine is typically at least 50-60%. 437 F.3d at 75 (citing Eli, 379 F.3d at 1021); see also Eli, 379 F.3d at 1021 (<HOLDING>). 7 . Powell noted that a characteristic of
holding that the evidence was sufficient to support a conviction under  856 where premises contained two white envelopes containing thirtytwo packs of crack cocaine equipment required for the manufacture and packaging of crack cocaine fortyone white envelopes containing particles of crack cocaine and crack cocaine stored in a laundry bag in the bathroom holding two prior convictions for distribution of crack relevant to show intent to distribute the crack found in defendants possession holding admissible police officers expert testimony that seized cocaine of eightythree percent purity would be cut with lactose to achieve lesser purity prior to streetlevel distribution holding that purity of 3644 did not disqualify a drug as crack in light of the chemists testimony that he had previously tested crack of lowerthantypical purity holding that the defendants prior distribution of crack was admissible to prove that he had intent and knowledge regarding crack found in a gym bag