This appeal presents for our determination whether there was sufficient evidence presented by the prosecution at trial to support the jury’s verdict that appellant was guilty of possession of a controlled substance (cocaine) in violation of D.C.Code § 33-541(d) (1988 Repl.). We conclude there was and so affirm.
The basic facts developed at trial are uncomplicated. Police officers observed appellant drinking beer while seated in a public park.1 As the officers approached *1037him, they saw that he held a cigarette with its tobacco removed and end twisted,2 and that he put an object in his coat pocket. When the officers identified themselves, he arose from the bench and appeared to put an object in the beer can from which he had been drinking. A brief struggle ensued before appellant was handcuffed during which the can fell to the ground and some of its liquid spilled out. The officers recovered “a small off-white colored rock-like substance from the ground in a puddle of beer near the beer can” and “a small, white colored rock-like substance” from appellant’s coat pocket.
The officers performed separate field tests on each of the “rocks” they had recovered and both tested positive for cocaine. The rocks, one of which was larger than the other,3 were placed in separate and secure plastic bags and routinely transmitted to the Drug Enforcement Administration laboratory for chemical analysis by a forensic chemist. The chemist’s report was served upon counsel before trial and submitted to the jury as evidence. It certified that the “white chunks” weighed “144 mg.” and were 97% cocaine.
At trial the prosecutor put to the government’s expert witness a question whether “assuming the weight to be 144 milligrams with 97 percent of purity of crack ... is this a usable amount?” The expert answered, “A usable amount is any amount of a drug that can be taken in or ingested into the body. In this particular case it is 144 milligrams which is plenty to go ahead and put into a device and smoke it.”
At no time during the trial did the defense contend that the rocks were not usable as drugs, but rather it presented a witness who testified appellant never had possession of the rocks.
The trial court’s instructions to the jury created the difficulty which now divides this court in its decision of this appeal. Specifically, the court charged the jury: “Now the Government does not have to prove that there was any particular amount of cocaine involved, but it does have to show you beyond a reasonable doubt that there was a usable amount of cocaine possessed.” The court went on to say:
The evidence in this case suggests that there may be two ... elements of cocaine possessed; one allegedly possessed in the pocket of the defendant, the other allegedly on the ground. Now you may find the defendant guilty if you find that he possessed either of these pieces of ... alleged cocaine, but you must be unanimous as to which one he possessed. For example, if six of you found that he possessed the one allegedly taken from his pocket and six found that he possessed the one allegedly in the puddle, but you could not agree on which one, you would not be able to find the defendant guilty. You would have to be unanimous as to at least one that he possessed in order to find him guilty.4
The jurors after some deliberation submitted two questions to the judge. The first question was: “In order to find the defendant guilty, if the jury can agree on only one of the specimens of cocaine belonging to the defendant, does this single specimen have to constitute a usable amount?” The second question was: “If the answer is yes, do the specimen on the ground and the specimen in his pocket each *1038constitute a usable amount? If only one does, which specimen is it?”
The judge answered the first question in the affirmative. As to the second question he advised, “I can’t supply you with an answer to this question. You will have to use your own memories and your own judgment.” Thirty minutes later, the jury rendered its guilty verdict, which appellant now challenges.
Appellant argues in essence that while there was direct evidence in the forensic chemist’s report and the expert’s testimony that the two rocks together were usable as a narcotic, there was no direct evidence in either the report or the testimony that each rock was in and of itself usable as a narcotic. Thus, the issue is whether the jury’s ultimate conclusion in this ease — under the particular instructions given — constituted a reasonable inference from all the evidence or amounted to an improper, and hence reversible, speculation without evidentiary foundation.
This court, in Wishop v. United States, 531 A.2d 1005 (D.C.1987), stated with respect to the evidence required to prove that a proscribed substance is usable:
Edelin [v. United States, 227 A.2d 395 (D.C.1967) ] holds only that if the quantity of a drug is too small to be capable of quantitative analysis, there must be “additional proof of its usability as a narcotic” in order to sustain a conviction. 227 A.2d at 399. Implicit in this holding is the converse proposition that if the quantity is more than a trace, and therefore measurable, “additional” proof may not be necessary (though of course it is not prohibited, and in some cases the facts may require it.)
Wishop v. United States, supra, 531 A.2d at 1008. This court went on to state:
Edelin and its progeny make clear that the critical requirement is usability, not measurability.... [T]he fact that a drug is measurable — i.e., capable of quantitative analysis — will usually suffice to prove that it is usable.
Id.
In the instant case, the rocks of cocaine were capable of and subjected to quantitative analysis. The laboratory test found that they weighed 144 milligrams and were 97% pure cocaine. The expert witness testified that their weight exceeded the minimal amount sold in the street, viz.: 70 milligrams, and were “plenty” to smoke as a narcotic drug. Thus, the instant case contrasts sharply with Singley v. United States, 533 A.2d 245, 247-48 (D.C.1987), upon which appellant relies. There, the government was able to present evidence only that “a ‘small amount’ ” of heroin was in the substance seized from the defendant, and the expert witness was unable to state an opinion whether these “traces” or “residue” of heroin were usable as a narcotic. As a consequence, the prosecution was forced to rely on its proof of the usability of the substance as a narcotic upon the prospective use of the substance by the defendant. Given this set of circumstances, this court was compelled to conclude that the jury had to speculate in reaching its guilty verdict and, hence, the conviction could not stand.
Appellant, as well as the dissent, urge us to conclude that there was simply no evidence from which the jury could find that each rock was usable as a narcotic drug and hence the trial court erred in refusing to grant his post-verdict motion for judgment of acquittal. Given the forensic evidence presented that each rock was almost pure cocaine and together weighed 144 milligrams,5 that each rock was of such size that it could be held, that only 3% of the substance (an amount significantly less than even the smaller rock) was not cocaine, that the amount of cocaine in the rocks according to the expert was “plenty” to smoke, and that the government is entitled to all reasonable inferences in its favor *1039when a challenge to the sufficiency of the evidence is made, Curry v. United States, 520 A.2d 255, 263 (D.C.1987), we conclude that there was evidence to support the jury’s verdict, and the judgment must be and is
Affirmed.
. D.C.Code § 25-128(a) (1981 & 1990 Supp.) proscribes drinking or possessing open containers of alcoholic beverages in, inter alia, parks.
. There was testimony that crack cocaine can be broken up and placed inside a regular cigarette for smoking after the smoker has first removed some of the tobacco.
. The trial court, in denying appellant’s post-trial motion for judgment of acquittal, stated “[e]ach rock could be seen and held ... [tjhough one rock is significantly smaller than the other.”
. As the trial judge recognized and stated subsequent to giving the charge, "the unanimity instruction ... probably need not have been given.” We agree. Appellant’s alleged possession of two rocks of cocaine, one in his pocket and one on the ground in close proximity to him, constituted a single legally cognizable act of possession. Briscoe v. United States, 528 A.2d 1243 (D.C.1987). Appellant did not offer at trial differing defenses to possession of each of the two separate rocks of cocaine. See Scarborough v. United States, 522 A.2d 869, 873 (D.C.1987) (en banc).
. We note this court’s comment in Wells v. United States, 515 A.2d 1108, 1114 (D.C.1986):
The government is not required to quantify the exact purity of the heroin in each package as long as the substance seized consisted of more than mere traces of heroin and can be identified as heroin.
In the instant case, the evidence was that the white chunks consisted of cocaine and were more than mere traces — 144 mg.