Investors continued to pile into some Korean stocks seen as related to superconductors amid claims about a breakthrough in the technology, even as the exchange warned of speculative bets and unfair trades.

Small caps including Duksung Co. and Sunam Co. jumped by as much as their 30% daily limits for a third-straight day. Sunam has skyrocketed about 260% in the past six sessions while Duksung is up 170%. Mobiis Co. also soared by as much as 30% Thursday, while Shinsung Delta Tech Co. briefly climbed 21% to a record high before erasing its advance.

Korea Exchange asked investors late Wednesday to be wary before investing in Duksung, Mobiis and Shinsung Delta Tech. It issued the lowest of three levels of warnings on the shares, stopping short of halting trading.

The bourse hands down such designations when there’s a probability of speculative bets and unfair trades so that investors may exercise caution before investment. The exchange operator can escalate warning levels before mandating a one-day trading suspension.

Stocks seen as tied to superconductor technology have been rising since a July 22 report from South Korean researchers stating that they had synthesized the world’s first superconductor able to conduct electricity at room temperature and ambient pressure. The superconductor utilizes a lead-based material dubbed LK-99.

Read: Superconductor Breakthrough Buzz Spurs China, Korea Tech Rally

The rally fizzled in China on Thursday. Jiangsu Fasten Co. dropped by as much as the 10% limit in Shenzhen after the company said it’s a metal products maker and environmental protection firm, and hasn’t conducted research into the technology. Henan Zhongfu Industry Co. also fell by the limit after clarifying that while it took part in a 2010 superconductor project run by the Chinese Academy of Social Sciences, a state-run think tank, it only provided the venue and equipment.

“Investors should be cautious on increased volatility in these superconductor theme stocks as their substance is not clear,” said Han Jiyoung, an analyst at Kiwoom Securities Co.

Elsewhere, American Superconductor Corp. dropped 29% Wednesday after surging 129% over the previous five days. Japanese wire makers that had gained recently on the superconductor trade fell Thursday after disappointing earnings from Sumitomo Electric Industries Ltd.

--With assistance from April Ma.