Garnet Types & Demantoid

Garnet Types and Characteristics

• Topazolite: a pure yellow andradite
• Spessartine: pale yellowish orange to dark brownish red, with a highly saturated, reddish orange hue and medium to medium-dark tone
• Malaya: a pinkish orange mix of pyrope, spessartine, and almandine, often referred to as "peach" or "cinnamon"
• Hessonite: an orange to cinnamon-colored variety of grossular garnet with a brownish hue
• Rhodolite: a range of dark, purplish raspberry red to light reddish purple, with a medium to medium-dark tone and strong saturation

Garnet Chemical Composition

• Garnet: basic composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible
• Tsavorite: a trace element of vanadium
• Demantoid: a trace element of chromium and iron
• Spessartine: a trace element of manganese and iron
• Hessonite: a trace element of manganese and iron
• Pyrope: top stones have a trace element of chromium and iron, with less vibrant hues
• Spinel: intense reds and pinks are caused by traces of chromium, while orange and purple stones owe their color to a mixture of iron and chromium

Forming Sources

• Garnets: formed in metamorphic rocks and pegmatite veins
• Demantoid: formed in volcanic and metamorphic rocks
• Spessartine: formed in pegmatites and metamorphic rocks
• Rhodolite: formed in alluvial deposits and metamorphic rocks
• Almandine: formed in metamorphic rocks like schist
• Spinel: formed in marble and alluvial deposits

Crystal Structure

• Garnets: all have a cubic crystal system, with the most common habit being dodecahedron
• Tsavorite: rarely forms well-formed crystals, often in irregular nodules called "potatoes" or "pods"
• Spinel: has a cubic crystal system, with a crystal habit of an octahedron, and can form flattened twin crystals

Clarity and Inclusions

• Garnet and Spinel: are singly refractive
• Tsavorite: has an extinction in darker stones
• Demantoid: has curving, radiating bundles of fibers known as "horsetails", which can increase the value of a stone
• Hessonite: most hessonite is highly included, with inclusions causing an instantly recognizable "heat wave" or "scotch-in-water" effect

Garnet Species

• There are 20 garnet species, with only 5 commercially imported as gems.
• Many garnets are mixtures of two or more garnet species.

Demantoid

• A variety of andradite garnet.
• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Trace elements of chromium and iron.
• Found in volcanic and metamorphic rocks.
• Rarely forms well-shaped crystals, often occurs in irregular nodules called "potatoes" or "pods".
• Crystal system is cubic.
• Singly refractive.
• Characterized by curving, radiating bundles of fibers known as “horsetails”.
• Often used in pave settings, rarely found over 3ct.

Tsavorite

• A variety of grossular garnet.
• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Trace element of vanadium.
• Found in metamorphic rocks and pegmatite veins.
• Rarely forms well-shaped crystals.
• Crystal system is cubic.
• Singly refractive.
• Calibrated squares, rounds, and ovals under one carat are popular.
• Larger stones (7×5 mm and larger) are more rare.

Spessartine

• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Trace elements of manganese and iron.
• Found in pegmatites and metamorphic rocks.
• Can be cut large without losing brilliance or color.
• Often cut into calibrated shapes, including rounds, ovals, octagons, squares, hearts, pear shapes, and trillions.

Rhodolite

• A mixture of pyrope and almandine garnet.
• Found in alluvial deposits and metamorphic rock.
• Good for being calibrated, prices rise steeply at 10ct and above.
• Often cut into free form cuts by lapidary designers.

Pyrope

• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Trace elements of chromium, iron.
• Found in metamorphic rocks.
• Small red brilliant-cut or rose-cut pyropes are common.

Almandine

• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Trace elements of iron.
• Found in metamorphic rocks like schist.
• Often cut as a cabochon with a curved undercut bottom, known as a "hollowback".
• Reflective foil backings can improve their dark appearance.

Pyrope-Almandine Mix (not Rhodolite)

• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Calibrated shapes, including rounds, ovals, octagons, squares, hearts, pear shapes, and trillions.
• Larger carats have darker tone and are often cut to shallow proportions.

Hessonite

• A variety of grossular garnet.
• Chemical composition is silicon, oxygen, and aluminum, with isomorphous replacement being possible.
• Trace elements of manganese and iron.
• Found in metamorphic rocks.
• Often too included to be used in jewelry, crystal inclusions—usually apatite, diopside, and zircon.
• Characterized by an instantly recognizable “heat wave” or “scotch-in-water” effect.

Spinel

• Chemical composition is oxygen, magnesium, aluminum, and silicon.
• Intense reds and pinks are caused by traces of chromium.
• Orange and purple stones owe their color to a mixture of iron and chromium.
• Found in marble and alluvial deposits.
• Crystal system is cubic.
• Singly refractive.
• Formatted into octahedral crystals, can form flattened twin crystals.
• Flux-grown synthetic spinel is also manufactured into triplets.
• Bright, very intense pink spinels command high prices amongst collectors.