Black Diamond is one of three closely related features, the others being Black Opal Pool and Wall Pool, occupying craters that, according to Bryan (TGoY), "definitely did not exist before the early 1900s." Extensive, and sometimes very powerful, hydrothermal explosions between 1902 and 1934 created the three features, notably "a series of titanic steam explosions" (Bryan) in 1934 that created Black Opal Pool. Although small vents on their periphery remained active for a time, Black Opal and Wall have not been known with certainty to have been active since the 1950s. Black Diamond, however, remains an active, if infrequent, geyser; its most recent recorded eruption was on or around 22 April 2016.
Richard Powell made a careful study of Black Diamond's activity from 2006, when it reactivated, through 2011, that is recorded in the GOSA Transactions, the technical journal of the Geyser Observation and Study Association (Powell, "Black Diamond Pool Eruptions 2006 - 2011," Transactions vol. XII, pp. 66-83). His study includes observations by Park staff and visitors as well as many of his own. Black Diamond's eruptions, though usually brief, can be very powerful, and many eruptions have been deduced from debris thrown around the crater area, dead birds(!), etc. For a time, Ralph Taylor maintained data loggers at Black Diamond under an NPS research permit, a difficult technical challenge as the thermistors tended to simply get thrown out of the pool when there was an eruption! It is unclear whether a logging capability still exists for this feature.
Eruptions can reach heights greater than 50 feet, although the initial hydrothermal explosions were considerably more powerful. The water is usually very muddy and rocks and other debris are expelled in the eruption. Intervals are long (usually) and erratic, and no periodic behavior is known.