speculate in (something)
To buy or sell some asset, such as commodities, securities, property, etc., at a risk while hoping to gain a profit from short-term market fluctuations. My uncle made, then promptly lost, a fortune speculating in oil wells in the 1980s. You'd be a fool to speculate in dairy in today's market.
speculate in something
to make risky business deals in the buying and selling of something. Jeff made a fortune speculating in cotton. I do not wish to speculate in anything. It is too risky.
speculate on something
to make a hypothesis about something. I really don't want to speculate on what might happen next. Would you care to speculate on what might happen if you quit your job?