|Garnet var. Hessonite (Grossular)
is a calcium-aluminium species of the garnet group of minerals. It has
the chemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be
replaced by ferrous iron and the aluminium by ferric iron. The name
grossular is derived from the botanical name for the gooseberry,
grossularia, in reference to the green garnet of this composition that
is found in Siberia. Other shades include cinnamon brown (cinnamon stone
variety), red, and yellow. Grossular is a gemstone.
Grossular should not be called grossularite, grossularite was once a type of rock.
Hessonite or Cinnamon Stone, is a more common variety of grossular with
the general formula: Ca3Al2Si3O12. The name comes from the Greek hēssōn,
meaning inferior; an allusion to its lower hardness and lower density
than most other garnet species varieties.
It has a characteristic red color, inclining to orange or yellow, much
like that of zircon. It was shown many years ago, by Sir Arthur Herbert
Church, that many gemstones, especially engraved gems (commonly regarded
as zircon), were actually Hessonite. The difference is readily detected
by the specific gravity, that of Hessonite being 3.64 to 3.69, while
that of zircon is about 4.6. Hessonite has a similar hardness to that of
quartz (being about 7 on the mohs scale), while the hardness of most
garnet species is nearer 7.5.
Hessonite comes chiefly from Sri Lanka and India where it is found
generally in placer deposits, though its occurrence in its native matrix
is not unknown. It is also found in Canada, Brazil and California.