These crystals grew where magma met limestone.
A body of diorite pushed up into limestone in the eastern Sierra de Cruces and cooked the rock it touched. Hot fluids carrying silica moved out of the intrusion and into the carbonate, and the two chemistries reacted along the contact to build an entirely new rock. Geologists call that rock a skarn, and it is where a large share of the world's best garnet crystals come from.
The garnets grew inside that reaction zone alongside quartz, calcite, phlogopite mica and vesuvianite. They are rarely much larger than a centimetre here, which is part of why sharp ones are collected rather than cut.
Nothing about the colour is added. The red comes from trace manganese sitting in the crystal structure where calcium would normally be.
Mineral: Grossular garnet (Ca₃Al₂(SiO₄)₃), a calcium aluminium silicate
Crystal system: Cubic
Origin: Sierra de Cruces, Sierra Mojada Municipality, Coahuila, Mexico
Hardness: 6.5 to 7 on the Mohs scale; hard, but the crystals are brittle at the edges and chip rather than scratch
Treatment: None
Dimensions: 2.3 x 3.03 x 2.64 cm
Weight: 17.17 g
You will receive the exact piece photographed.