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No sooner than Kanji were being introduced to Japan, Japanese-made Kanji, known as 国字 or 和製漢字, in Japanese, were being created. 国字 have come and gone. By the beginning of the Heian Period (794~1185 A.D), around 400 such characters were recorded in the 新撰字鏡(しんせんじきょう), an early 漢和辞典. Although this lesson won't go over hundreds of individual such Kanji, you will learn more about those that are most commonly used today.  

The majority of 国字 are ideograms (会意文字), meaning that they are composed of more than one part with meaning being derived from the parts being combined.

As an example, the sacred tree in Shintoism known as the sakaki tree is written with the Japanese-made 榊, which is a combination of 木 (tree) and 神 (god/kami). 

This is contrary to Kanji overall which are typically phono-semantic compounds (形成文字) in which one part indicates meaning and one part indicates sound. This is because 国字 were predominantly coined to represent Japanese words for which there was no suitable existing Kanji. 

As another consequence of this, most 国字 only possess KUN reading(s), but some exceptional ON readings do exist for 国字 which have been borrowed into Chinese. For example, the Japanese-made 働 meaning "to work" has both the KUN reading はたら(く) and the ON readingドウ. There are even some 国字 which only possess an ON reading because the Kanji itself was formed as a phono-semantic character with the word usually having a complicated etymology based on Sino-Japanese phonotactics. An example of this is 鋲 with the ON reading ビョウ meaning "thumbtack."  

While there are still hundreds of extant examples of Kokuji in Modern Japanese writing, only nine such Kanji are in the Joyo Kanji List (常用漢字表). These Kanji are as follows:

As mentioned, the majority of 国字 were created by combining one or more parts for their meanings to express a word for which no suitable Kanji existed. More examples of this include the following: 

While it isn't difficult to find lists of 国字, it may be difficult to find ones that are most beneficial to learn. Since some are far rarer than others, the table below ranks them on a scale of 1-10 of importance with 10 being most crucial to 1 being least important. If one of your ambitions is to pass the Kanji Proficiency Test Level 1, then you would need to try to learn as many as possible.