The word garnet comes from the latin expression "granum". That means grain and is a hint to the shape of he crystal which is often a rhombo dodecaedra (cubic system). If itīs not perfectly crystalized or damaged (tumbling in a river) it looks like a small ball with fragments of regular edges.
The hardness is 7-7,5, the chemical structure is a mess.
There are several varieties of garnet, hereusual suspects:
Almandine: red, brownish red, redish black.
Andradite: bright green (demantoide), brownish yellow, brown, black.
Grossulaire: white, grey, yellow, green(tsavorite/Cr gross), pink, brown,
Pyrope: bloodred, pinkish red, black, dark to light violet (rhodolite).
Spessartine: orange (mandarine garnet), yellow, redbrown.
Uwarowite: dark to light green.
Any of these stones can have colors inbetween the listed.
There is also a variety named andadite from Mexico which shows various colors (Rainbow garnet).
Normally garnets grow in mixed crystals, a pure piece out of only one variety is very rare.
There is also a pyrope/spessartine/(almandine/grossulary) mixed garnet from madagascar, tanzania and sri lank which shows a strong color change from a greenish blue in daylight to a redish violet in electric light. This effect is depending on a 0,2-3% share of crome and/or vanadium. If there is only low share of Cr and V its also possible that the changing goes from yellow to pink (extremely rare).
Garnets appear in various varieties, colors and "special efects" like chatoyance or asterism, but if a 7-ray star, clear mandarine garnet, Mohs hardness 12, with a strong colorchange is offered to you, be careful, maybe itīs oiled ;-).