Mandarin garnet is a bit imprecise, especially as I understand you
were given a challenge.
If mandarin garnet is a reference to the colour, than it is not that
difficult and search on “spessartite” or “spessartine” should yield
some results. If “mandarin” used as a trade name, it refers to the
garnet which comes from Namibia and known to gemologists as
"hollandine". Hollandine crystals as a rule do not exceed 10
millimeters in size, so cutting a 5 carat stone is out of the
question. Namibia has another source of “mandarin garnet” which comes
from Kumene River. This garnet has another trade name of Malaia
(Malaya) and 5 carats size is not restrictive. You have to be very
clear if Malaia garnet would satisfy the condition of your challenge.
While “mandarin” and “malaya” used interchangeably they are quite
different. Malaia is a mixture of spessartine and pyrope, while
hollandine belongs to grossularite garnets. Another name you may
encounter is a hessonite or essonite garnet. Since portuguese cut was
mentioned, which is highly unusual for garnets, I think that
hessonite may be the stone you are looking for. Hessonite can exhibit
a “heat-wave” effect and portuguesse cut is a good way to minimize
If it is a hessonite you are looking for, the condition of the size
of the stone may be impossible to meet.
It is critical that you understand the difference between hessonite
which is commonly available, and hessonite (hollandine) from that
particular location. I have also seen hollandine and spessartine used
as it would be the same stone, even in publication which one would
think should know better, so clarify with the person who gave you the
challenge if this distinction is important to him.