It is well known the source of the majority of gold in the western United States is from the ancient dead rivers often called the "Tertiary Channels." Many don't know diamonds have also been found in the tertiary gravels.
The ancient tertiary rivers which originally drained the landscape drained a massive area. Later these rivers were buried by volcanic activity and capped with lava. Extensive areas of what is now California were buried deep under lava. Remnants of these volcanoes can still be seen at places such as Mt. Lassen.
Diamonds are associated with volcanic activity. Not all vocanic areas may contain diamonds, but it rare to find diamonds in areas not formed by volcanoes. Many areas of California hold diamonds. These gems are usually yellow or pinkish in tint and its likely you've thrown a fair number of them back into the creek. Many of these diamonds are small, and mixed in with your black sand concentrates.
Some of the earliest geologic surveys of California reported the presene of diamonds, but these diamonds were normally found purely by chance. A raw diamond looks little like the polished gem you see in jewelry stores and quite often looks like a dull piece of quartz.
In The Auriferous Gravels of California, 1880, Whitney provides a thorough description of how and where these diamonds are found. Whitney writes "The number of localities where the gem has been observed is considerable. The followin is a list of these occurences which has been brought to the attention of the Geological Survey:
At the McConnel and Reed claim was found a colorless and brilliant diamond about three to four feet above the bedrock in the overburden, it was about the size of a small bean. Mr. McConnell believes he had previoulsy thrown away a similar diamond the size of the end of his thumb, not recognizing what it was.
In the possession of Mr. Robert Cruson was a diamond measuring nearly one quarter inch which was found while cleaning out the sluices. It weighs about 1.5 carats. "I have little doubt that a good many have been picked up here, looked at an thrown away."
At the mines near White Rock diamonds were found in the sluice box which had a slightly yellowish tint and the largest was nearly one half carat.
Diamonds have also beeen found near Jackass Gulch, near Volcano; Indian Gulch; Loafer Hill near Fiddletown from which quite a number have been found.
French Corral, in Nevada County, has produced a number of diamonds. At Cherokee Flat, in Butte County fifty six diamonds were picked out of the gravels at various times.
In El Dorado County diamons have been found on the south side of Weber Hill, White Rock Canyon, Dirty Flat and Smith's Flat.
In Amador County diamonds have been found in Jackass Gulch, Rancheria and Loafer Hill.
In Butte County diamonds have been found near Cherokee Flat and Yankee Hill.
In Plumas Couny diamonds have been found near Gopher Hill and upper Spanish Creek.
In Sierra County diamonds have been found in the Laporte Mining District.
Numberous diamonds have been found in the northwestern part of California. Locations are shown on the below map.
For further reading on diamonds you can check out DiamondProspector.com