Biscuit-like sinter deposits once lined the edge of Sapphire's crater, and in the 1880s it received its name for the knobby formations. The 1959 earthquake caused Sapphire to erupt, breaking and dislodging the formations. Biscuit Basin is an isolated thermal group and is actually a part of the Upper Geyser Basin.

Biscuit Basin contains a small collection of thermal features. Many, however, are small, gem-like encrusted pools and geysers, including Silver Globe Spring, Sapphire, and Black Opal pools, Jewel, Cauliflower, and Black Pearl geysers. The Firehole River and a highway divide the basin. A smaller group, located east of the river, contains mainly hot springs. Cauliflower Geyser is the main feature of this group, and it is identified by the cauliflower or biscuit-like sinter masses surrounding the crater.

Sapphire Pool dominates the main group west of the river. The water of this pool, or spring, is crystal clear with a Oriental blue sapphire tint. Other important features include Shell Geyser, which has a golden-lined crater, and Jewel Geyser, known for the shiny, beaded sinter around its vent.

RUSTY GEYSER
Temperature 202°F Interval 2-3 minutes. Duration 20-45 seconds. Height 4-6 feet. This small geyser received its name for its rusty, red-colored basin surrounding its funnel-shaped vent. Iron oxides are responsible for staining the sinter. This thermal feature was dormant before the 1959 earthquake. Since then, except from 1964-1967, it has been an active and frequent spouter. Its temperature is above the boiling point and this may be one reason why it is a steady geyser. The eruptions are spontaneous. Water splashes violently for the first ten seconds and then declines gradually in activity. Fumaroles are present around Rusty, but no known underground connection exists with any other thermal feature.

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SAPPHIRE POOL
Temperature 200-202°F Dimensions 18x30 feet. Sapphire Pool, named for its blue, crystal-clear water and for its resemblance to an Oriental sapphire, was once a placid hot pool. It was not until after the 1959 earthquake that major eruptions occurred. For several years following the earthquake powerful eruptions at two hour intervals reached 150 feet. The force of the eruptions caused the crater to double in size, destroying the biscuit-like formations around its edge, and the crystal-clear water became murky. By 1968 Sapphire ceased to function as a true geyser. Today Sapphire still retains its crystal-clear, blue water, and still violently boils and surges occasionally.

JEWEL GEYSER
Temperature 199°F Interval 5-10 minutes. Duration 60-90 seconds. Height 10-30 feet. Its name is descriptive of the pearl-like sinter beads formed around the vent. Soda Geyser was the original name given by the Hayden Expedition, but turn-of-the-century visitors changed the name to Jewel Geyser. Jewel has frequent and regular eruptions. Before an eruption the vent suddenly begins to fill with water and churns to overflowing, triggering eruption. A burst or jet of water projects 15-30 feet high and collapses, followed by a quiet pause. An eruption consists of a series of one to five separate bursts. Jewel does not appear to have underground connections to other thermal features but may have some connection with Sapphire Pool.

SHELL GEYSER
Temperature 200°F Interval 1.5 to several hours. Duration 20-90 seconds. Height 5-8 feet. The golden sinter lining of the crater resembles the shell of a bivalve, hence the name. Shell Geyser is very irregular and the interval between eruptions changes from year to year. Before an eruption, water in the crater begins to rise and may boil. Heavy churning then occurs, setting off the first small, weak eruption. As the eruptions subside water begins to lower and drain back into the crater. No underground connections are known to exist with other thermal features.

AVOCA SPRING
Temperature 199°F Interval 1-18 minutes. Duration 10-30 seconds. Height 10-20 feet. The Hague Party in the late 1880s named this spring with a three-foot-diameter crater. It was not until after the 1959 earthquake that it became an active geyser. Since then it has been a frequent spouter. The fountain-type geyser, located on a rise above the other thermal features at Biscuit Basin, has occasional changes in interval and duration. When an eruption occurs, waterjets in several directions from a filled crater. There is a pause between bursts, but the water continues to churn. As the eruption subsides, water drains from the crater. There may be subterranean connections with the Silver Globe group.

MUSTARD SPRINGS  
Temperature 172-198°F Interval 5-10 minutes. Duration 5 minutes. Height 4-6 feet. Two springs, East Mustard and West Mustard springs, make up this group. The springs, separated by 50 feet, are eight to ten feet in diameter and resemble each other in shape and size. They received their name for the mustard-colored lining of their craters. Although past earthquakes have changed their status several times from geysers to springs, both have erupted. West Mustard Spring was the most active between 1961-1983. A tremor in 1983 reversed this and now East Mustard Spring is a true geyser, and West Mustard is an inactive, apparently dry, spring. The two springs have subterranean connections.