California Fractional Gold come in 3 sizes (25c, 50c, $1) and 2 shapes (round and octagonal). All are denominated, meaning that it says dollar or cents on the coin, or an abbrevation of that.
Many of the major types are pictured below

California gold refers to gold coins that originated due to a shortage of U.S. coinage during the first few years of the California Gold Rush. Their small size can be both attributed to the fact that they were supposed to be worth thier denomination in gold, and also made to approximate a pinch of gold dust. The research has shown that they hardly circulated as actual coinage after 1856 or so.

Those first Cal Gold varieties, dating 1852-1856 are commonly referred to as Period 1 coins. These are well documented as to having circulated as coinage. Period 2 coins were made from the late 1850's to 1882. Their purpose was mostly as souvenirs, etc. [ One should note that at the same time, some jewelers who made these denominated California Gold coins, also made Cal-Gold tokens that were not denominated. These are also quite collectable and were produced on into the early 1900's. ] Then, you also have what's referred to as Period 3 coins. These are a few denominated varieties that were made, without a doubt as souvenirs, in the late 1800's, and probably early 1900's.

The denominated pieces, for the most part, ceased production in 1882 because of the person, Colonel Henry Finnegass. His job was to track down and melt as many of the denominated Cal-Gold coins he could find. This is a major reason why California Gold is so rare. Another telling reason for the rarity of California Gold is because the dies would wear out very quickly, and they were often reworked. This made for lots of different varieties.

As for content, that varies depending on who made the piece and when. It ranges from solid gold (9k on up) to gold plated with a lower grade gold core, or even silver or copper cores! California gold collectors don't collect them based on gold content, and neither is it a factor in determining the value of individual varieties.

You may be wondering at this point why California Gold isn't higher priced than it is considering how rare the coins are. Among the reasons for this is that they weren't made by the US mint, they've had some bad publicity in the early 1990's from Telemarketers over-pricing them, and lastly there are a number of modern reproductions that confuse people.

When you consider that there are roughly 600 different varieties of denominated pieces, and twice that many of the tokens, knowing what's what is difficult. For the denominated pieces, there is a new edition out of the Breen-Gillio book on these coins, and is available at Bowers & Merena. I highly recommend buying that book before venturing forth into this area of collecting.