The Rare Earth Oxides Group is sometimes referred to as just the Rare Earths or sometimes as REO. This group of minerals is purely an informal group as most official mineral groups are related first by structural similarities and secondly by chemistry. There are no structural characteristics to this group as a whole. The reason for this group is because of the chemical and general physical property similarities that exist in most rare earth oxides.
First, the chemical similarities. All members of this group are members of the Oxide Class and contain, as a significant portion of their chemistries, members of the so called rare earth elements. Rare earth elements or rare earth metals are all part of a series in the Periodic Table of Elements called the Lanthanide Series that runs from atomic number 57 to 71. In addition the elements yttrium #39, thorium #90 and scandium #21 are also considered to be rare earth metals because they share similar properties. Uranium, although often associated with rare earth metals, is not technically a rare earth metal.
There are many uses for rare earth metals. They are constantly being used in the nuclear power and nuclear weapons industries for both practical and experimental utilization. Some rare earth metals have found more down to earth applications in metallurgy, ceramics, glass making, dyes, lasers, televisions and other electrical components. The movie industry makes use of several rare earth metals to help produce the high intensity lamps that are needed for movie projectors. Yttrium has been used for decades in a diamond simulant called YAG or yttrium aluminum garnet which has physical properties similar to natural garnets but with a brilliance and fire more similar to diamond. The importance of rare earth metals is most certainly on the rise.
The rare earth metals were given this nickname because they were thought to be extremely rare in the Earth's crust. Now many of them are thought to be as common an element as silver. Dispite this, as the label implies, these metals are rare in the Earth's crust or more accurately they are rarely concentrated enough to be mined in large quantities. The reason for this rarity of concentration is that these elements are incorporated into other commoner minerals as mere traces with far too low of concentrations to be used as ores. There is no good concentrating effect as there exists with other valuable metals. Oxygen seems more capable than other negatively charged ions to round these metals up when they are concentrated above normal levels. This makes the rare earth oxides all that more valuable.
The rare earth oxide minerals share some general physical properties. They tend to be black or dark brown in color with reddish, yellowish or more commonly brownish streaks. They are nearly opaque to opaque with a few exceptions of translucent specimens. A variation of resinous, pitchy or submetallic luster is usually found within each rare earth oxide's range of luster possibilities as these lusters are not too different from each other. Rare earth oxides are relatively hard (5 - 6.5). They commonly have a very characteristic conchoidal fracture. Their higher than average specific gravities within each species range considerably from 3.5 to 7 and up; the variation is due to the large amount of substitution of constituent elements that these minerals allow. And finally, rare earth oxides will usually have a coating of an earthy surface alteration, which should not be removed if present as it is part and parcel of the mineral. As can be guessed, it is sometimes difficult to distinguish most of the rare earth oxides from one another, but identifying them as rare earth oxides is fairly easy.