Romulan mines in the 22nd century were armed with tricobalt explosives. Upon detonation, they produced a thermokinetic explosion equivalent to one quarter of a kiloton. These anti-spacecraft explosive devices, utilized by the Romulan Star Empire, were cloaked with primitive cloaking devices similar to those utilized by the Suliban of the same era, but operating on a different phase variance. The mines were similar in design to Triton-class spatial torpedoes used by United Earth around the same time. (ENT: "Minefield") The power matrix of the mine used bulky and primitive boridium power cells. (ENT: "Babel One")
In 2152, Enterprise NX-01 encountered a field of these mines, which were cloaked, surrounding a planet that the Romulan Star Empire had annexed. The explosion of one such device caused a significant amount of damage to Enterprise's port forequarter. Another mine lodged itself on the hull, dangerously close to the ship's starboard impulse reactor, but failed to detonate. Lieutenant Malcolm Reed and Captain Jonathan Archer attempted to disarm the mine, which Reed noted was possibly pockmarked by micrometeorites, but were unable to do so before two Romulan Birds-of-Prey arrived, demanding that Enterprise leave the system. Archer and Reed managed to separate the section of hull plating where the mine was attached to the rest of the ship, and detonate the mine at a safe distance from the vessel. (ENT: "Minefield")
In the final draft script of "Minefield", the Romulan mine was described as "a large, metallic spheroid" and the script went on to say, "We'll learn that this is an alien orbital mine [....] The mine's surface is faceted with octagonal panels. A number of javelin-like spikes protrude from the mine at odd angles." The script referred to the process of the spikes attaching to the hull of Enterprise as being done "magnetically." A subsequent script note stated that, while the quantum beacons are active, "Cloaked mines are not visible unless on the viewscreen." The script also referred to the yield of an exploding mine as "about fifty kilotons," rather than a quarter of a kiloton.
Regarding the explosive power of this mine, at a yield of 250 tons of TNT, the Romulan tricobalt mine was approximately twice as powerful as 23rd century cobalt bombs mentioned in the Star Trek: The Original Series episode "Obsession", said to be 1/10,000th of the yield of an ounce of antimatter or approximately 122 tons of TNT. 
As no parts of the Romulan mine were added with CGI in post-production and everything it did had to instead be shown with practical machinery, creating the mine turned out to be a challenging project. "We had two weeks to come up with this prop," recalled Set Decorator James Mees. "After much searching, I found a large, used fiberglass buoy, the kind used to mark channels for ships. We ended up cutting it open to add the Romulan 'electronics' and the spike that you see in 'Minefield,' along with the machinery to make it work." (Star Trek: Communicator issue 152, p. 35)