Not all words can - according to Arab grammarians - be etymologized, which means in practice: be reduced to a verbal root from which they derive.
Therefore, for example, while you will find most words in Arabic dictionaries subsumed under their respective root (e.g. "muslim" not under the letter "mim", but under the root s-l-m), other words are not subordinated under a root, because they cannot. Now, as fdb pointed out, some loans, once they are perceived as Arabic, can be squeezed into that pattern. Others are "left alone", perhaps not being important enough to generate a need for related words of the same general concept. Still they may undergo the same treatment as native Arabic words.
An example are the so called "retrograde singulars". From Greek yakinthos "jacinth, hyacinth" Arabic "یواقیت", "yawāqīt" was borrowed, therefrom a new singular "یاقوت", "yāqūt", was formed.