The Horizons Marijuana Life Sciences Index ETF ( HMMJ ) tracks companies' activities in the marijuana industry, and as of Sept. 5 it was above the $1 billion mark — Canadian. To be fair, that translates to a little more than $758 million in U.S. currency at the current exchange rate, but it is still considerable growth for an ETF that launched in April 2017, just a little over a year ago.

The investing market was abuzz this week with Fidelity Investments' new no-fee index funds reaching near-$1 billion in assets, but it took Fidelity two funds to get it done, while a single Canadian marijuana ETF passed the $1 billion market this week on its own. And it didn't even have the help of Friday's free weed-smoking advertisement from Elon Musk .

While marijuana stocks have been volatile, their performance has taken off in the past month since liquor giant Constellation Brands announced a $4 billion investment in marijuana company Canopy Growth. Molson Coors Brewing and Diageo also have taken stakes in Canadian marijuana companies.

HMMJ is up 43 percent in the past month, according to Morningstar.

The U.S.-listed weed ETF — ETFMG Alternative Harvest ETF (MJ) — was the best-performing ETF in August and is nearing $500 million in assets. Although up 28 percent this past month, volatility led to only a 6 percent year-to-date return. HMMJ is up 22 percent this year.

Some investing experts caution that these stocks are trading at multiples that reflect future, not current sales and earnings, and it is a difficult sector in which to make growth estimates. Cronos Group, the top holding in MJ and fifth-largest holding in HMMJ, is a current target of short-seller Andrew Left, who claims it is overvalued by two-thirds.

In Canada the federal legalization of marijuana is imminent, scheduled for Oct. 17. Last October, Constellation took a 9.9 percent stake in Canada-headquartered Canopy Growth. Canopy is the second-largest holding in HMMJ and MJ.

Overall, HMMJ holds 40 stocks, with a focus on medical marijuana, but it does reach with some of its holdings, such as fertilizer company Scotts Miracle-Gro (its sixth-largest holding), though Scotts Miracle-Gro did buy Sunlight Supply, the leading U.S. hydroponics distributor, last April. Several of its holdings are drug companies working on synthetic marijuana products, such as Insys Therapeutics, best known for its controversial opioid painkiller Subsys.

"Over the past year, we've come to better understand the cannabis market, the tremendous growth opportunity it presents and Canopy's market-leading capabilities in this space," Constellation Brands CEO Rob Sands said in a statement announcing the $4 billion investment.

"We think the premium paid as well as the size of [Constellation's] investment reflects the long-term attractiveness of the global cannabis opportunity," Cowen analysts said in a note analyzing the deal.

Horizon's management team said in a statement Friday that the assets "far exceeded our expectations" and pointed to the anticipation of Canada's upcoming recreational marijuana legalization. It also said that investors may be opting for ETFs over individual stocks since "nobody knows for sure which companies will truly be successful over the long term or even be swallowed up."