Katakana Analysis

I. カラット carat マージャン mahjong
カラット is a unit of diamond's mass first introduced from the West. In modern Japanese, katakana are most often used for trnascription of words from foreign languages. Since this word is from a western country probably using romanji, so it will be much easier to use katakana other than Kanji or hiragana. In China, this word is translated as 克拉 (kela) according to its pronounciation quite much the case as using Katakana. The point is that Chinese only have one writing system, Chinese characters, therefore they are written in Chinese charaters phonetically unless the author choose to use the foreign words directly for specific writing purpose. However, most loan words in daily use have been written in Kanji and are widely accepted by people however educated they are.
There is one interesting example here. In Japanese, America is written アメリカ Amerika. America also has its own Kanji Amerika (亜米利加) or for short, Beikoku (米国), which literally means "Rice Country". In Chinese, America is written 美国(meiguo). 美 indicated the phonetic segment "me" in America which literally means beautiful and 国 is Chinese word "nation". In recent times, young students also use 米国 especially in online conversation which probably is influenced by Japanese.
Although words borrowed from ancient Chinese are usually written in kanji, loanwords from modern Chinese dialects which are borrowed directly rather than using the Sino-Japanese readings, are often written in katakana. For instance, マージャン mahjong.
II.
Katakana has several usages such as loan words, onomatopoeia and empahsis. Therefore, it is not surprising to see different explanations focusing on different aspects of katakana. Meanwhile, in order to differentiate it with hiragana, some textbooks also describe it focusing on its writing styles such as in Genki.