Garnets are Red … Sometimes

January’s birthstone, garnet, is generally associated with the color red. But did you know garnets also come in orange, green, brown, and other colors?

Garnets are actually a group of gems with similar, but slightly differing chemical formulas.  The generic formula is A2B3(SiO4) where A is calcium, magnesium, iron or manganese (Ca, Mg, Fe, Mn) and B is aluminum, iron or chromium (Al, Fe,Cr). The different combinations of elements create the different species of garnet (listed below with their common colors). Color can be a very helpful diagnostic feature when identifying some (but not all) gems.  For instance, spessartine garnet must always have an orange color component.

min_garnet

Almandine garnet collected at Gore Mountain in New York (left), spessartine garnet crystal (locality unknown, right)

  • Pyrope: medium to dark reddish orange, red through slightly purplish red to pale pink, colorless
  • Almandine: brownish orange to brownish red to purplish red
  • Spessartine: orange, brownish orange, yellowish orange, reddish orange, and red-orange
  • Grossular: medium to dark orange, medium to dark green to yellowish green, colorless
  • Andradite: yellow to slightly greenish yellow or brown, green through yellow-green
  • Uvarovite: light to dark green through yellow-green to brownish green
  • Hydrogrossular: pink, green, brownish pink, white, gray

Because of the range of chemical formulas possible, garnets may also be mixes of more than one species.  In addition, some species have subspecies (called varieties).  Some examples are Demantoid (a variety of Andradite) and Hessonite (a variety of Grossular)

Some garnets also display “phenomena” such as stars, cat’s eye, and color-change due to features of that stone that affect how light is reflected and/or transmitted.

Other facts – Garnet: 

  • is the gemstone for those celebrating their 2nd wedding anniversary
  • is used as an abrasive
  • has been used in jewelry since ancient times
  • can be found in all three major rock types: igneous, sedimentary and metamorphic
  • has a hardness of 6.5-7.5 on the Moh’s scale

and comes in colors to please almost everyone!

GASSN102_a

Sources:

  • Gemological Institute of America, Gem Encyclopedia page on garnets: click here.
  • Gemological Institute of America, Gem Identification Lab Manual, 5/2012
  • Manual of Mineralogy, 20th Edition, Cornelis Klein and Cornelius S. Hurlbut, Jr., 1977.

Gem vs. Mineral

What’s the difference between a gem and a mineral?

Some, but not all, minerals are gems, and some, but not all, gems are minerals.  Confused?  Good, then we’re ready to start.

amethyst

Amethyst is both a mineral and a gem

A GEM has the following properties:

  • Rarity
  • Beauty
  • Durability

A MINERAL has these properties:

  • Inorganic
  • Naturally occurring
  • Set chemical formula or range of formula
  • Solid with unique crystalline atomic structure

(A rock is composed of more than one mineral although a large mass of a single mineral can also be called a rock.)

In general, the properties for gems are much more loosely defined than those for minerals.  Let’s look at some of these properties.

Rarity: Rarity describes how common a gem is and can change as gem deposits are mined out or new sources found. Rarity is also relative to each individual gem; for instance, large inclusion-free amethysts are common, but large inclusion-free emeralds are extremely rare. Generally, the “cleaner” and larger the gem, the rarer and therefore more expensive it will be.

Beauty: Not everyone agrees on what is beautiful, but there are gems for just about everyone’s taste.

Durability: Durability is a gemstone’s ability to withstand wear, heat, and chemicals. Durability consists of three properties: hardness, toughness, and stability. Hardness means how well a gemstone resists scratches and abrasion. Toughness describes how well a gemstone resists breaking and chipping. Stability means how well a gem resists chemicals and temperature changes. (http://www.gia.edu/diamond-care-cleaning)

Inorganic: Not a previously living plant or animal.

Naturally occurring: Not made by humans.

Set chemical formula or range of formula and unique crystalline atomic structure: This is what creates specific crystal shapes and consistent properties for a given mineral.

Examples of gems that are not minerals:

  • Pearls, fossils and amber (“organic” gems)
  • Any synthetic gem (not naturally occurring)

Examples of minerals that are not gems:

  • Gypsum, mica (not durable)
  • Milky quartz (not rare)
  • Hornblende (not beautiful as a gem)

When you look at a piece of gem jewelry, consider how these factors came together when you admire its beauty!