World Gemstone Resources - Strawberry Grossular Garnet from Mexico
Garnet is a very common rock-forming mineral. There are many varieties of garnet gemstones on the market, ranging from common varieties to rare varieties. Among them, grossular garnet is a common variety of garnet, but strawberry red grossular garnet is a relatively rare variety of grossular garnet. Its color ranges from pink to dark red. It is a calcium aluminum silicate mineral with a chemical formula of Ca3Al2[Si3O12].
PART 01
Origin and Discovery History of Strawberry Red Grossular Garnet
In 1974, a Mexican farmer accidentally discovered a small red crystal, which was identified as grossular garnet by a friend who was engaged in the stone trade. Later, as mineralogists and businessmen came one after another, the deposit was confirmed to be rich in pink to red rhombic dodecahedron grossular garnet crystals, white calcite and yellow vesperite crystals located in skarn matrix. Since the discovery of red grossular garnet, a large number of beautiful and attractive mineral crystal specimens have entered the collection market.
For decades, red grossular garnet has been produced in the Sierra de Cruces Mountains in Coahuila, Mexico, but some Mexican red grossular garnet crystal specimens are labeled as Lake Jaco, Mexico.
The origin of this place name is that when red grossular garnet was first discovered, some crystal merchants deliberately concealed its exact location information in order to maintain its rarity and prevent others from exploring and mining. They labeled these red grossular garnets as Lake Jaco in Chihuahua to the west. Lake Jaco actually crosses the border between Chihuahua and Coahuila, which led to confusion in the location information of specimens in the area during this period.
PART 02
Gemological Properties of Strawberry Red Grossular Garnet
Mexican strawberry red grossular garnet is pink to rose red in color, with black in the center of some samples, distributed in rings and stripes, with a clear boundary between the red edge and the black core area. Most crystals are translucent to opaque, highly euhedral, standard rhombic dodecahedron, glassy luster, and well-developed internal cracks.
In terms of size, the strawberry red grossular garnet with black core is larger than the strawberry red grossular garnet without black core. In terms of gemological properties, its refractive index is 1.741～1.755, and its relative density is 3.5874～3.6987. The relative density of the black core series grossular garnet is slightly higher than that of the grossular garnet without black core. The Mohs hardness is 6.96～7.75. The hardness of the black core area is slightly higher than that of the red edge, which is due to the difference in composition between the two.
PART 03
Geological origin and color-causing elements
This grossular garnet coexists with quartz, calcite, pyroxene, wollastonite and other minerals, and belongs to the skarn type deposit. According to its composition characteristics, it can be inferred that it was formed under medium temperature (450℃～700℃) and low pressure (500×1005Pa～800×1005Pa) geological conditions. In the initial iron-rich oxidation environment, Fe3+ isomorphously replaced part of Al3+ in the [AlO6] octahedral lattice to form the grossular garnet component, resulting in a black core area in the center of the crystal. As the environment gradually stabilizes, the Fe content decreases, and after the Mn ions are enriched to a certain amount in the lattice, the outer edge of the crystal appears strawberry red.