Rocks as Gems

The word “rock” doesn’t normally conjure up images of beautiful jewelry, but as I explained in my very first blog post (https://mochisgifts.com/2014/06/05/gem-vs-mineral/), some rocks are also considered gems. While many minerals meet the criteria of beauty, rarity, and durability to be considered gems, very few rocks do.

Because the conditions under which rocks form can vary greatly, there can be very different looking results. Even within a specific type of rock, although the main mineral components are the same, trace elements or other minerals, sizes of individual grains, relative amounts of each mineral, and more, may differ. Causes for these differences include: the elements available in the melt source, temperature, and pressure. Most of the rocks used as gems have individual minerals that are visible to the naked eye, but some, such as basalt (sold as “lava”) have mineral grains too small to see. This variability is part of the beauty of the rock, but can also make it very difficult to match pieces for jewelry.

Enjoy these examples of rocks that are gems – I’ve listed the main constituent minerals.

Lapis lazuli

Blue: lazurite
Brassy/gold: pyrite
White: calcite

Ruby in zoisite

Green: zoisite
Red: ruby
Black: hornblende

See my blog: https://mochisgifts.com/2019/07/03/ruby-in-zoisite/

Tiger Iron

Gray: hematite
Gold: tigers eye
Red: jasper

See my blog on hematite: https://mochisgifts.com/2014/10/17/hematite-synthetic-hematite-and-disclosure/

Unakite

Green: epidote
Pink/orange: potassium feldspar
Gray: quartz

Lava (basalt)

Individual grains too small to see but include:
Plagioclase feldspar
Pyroxene minerals
Olivene
and many other accessory minerals

Graphic Granite

(labeled as “feldspar” when I purchased it)
Gray: quartz
White: feldspar
and many other accessory minerals

So yes, we geologists love rocks, but now even non-geologists have a reason to enjoy them too!

Turritella Agate – A Snail By Any Other Name …

FTGFN105_a

Fossil turritella agate & garnet necklace (Item FTGFN105)

In last month’s blog I explained that for many gemstone beads, the country of origin may be difficult to determine.  Fossil turritella agate is an exception because it comes exclusively from the Green River Formation in Wyoming, USA.

Turritellas are marine snails (gastropods) with spiral shells.  When fossil snails were found in the Green River Formation, they looked like turritellas and were given that name.  However, it turned out that the snails were in fact an extinct freshwater variety and were renamed Elimia tenera.  By that time, the incorrect name had already become commonly used and the name elimia tenera has never managed to replace the incorrect turritella name.

Turritella fossils are among my favorite fossils to incorporate into my jewelry designs; the fossil snails are creamy to white spirals in a rich brown to almost black matrix.  The polish can be uneven on these stones due to the natural variation in the fossils, but that’s part of what makes each one unique.

The following is directly from http://geology.com/gemstones/turritella/:

“How did Turritella Agate Form?

About 50 million years ago, during the Eocene epoch, the young Rocky Mountains were almost finished growing, and the landscape of what is now parts of Colorado, Utah, and Wyoming consisted of rugged mountains separated by broad intermountain basins. Rains falling on the slopes of these mountains ran off of the land and collected into streams that carried sand, silt, mud, and dissolved materials down into the lakes that occupied the intermountain basins. Over time, these sediments began filling the lakes, and many types of fossils were preserved within them.

Abundant plants and algae grew on the margins of these lakes, providing a perfect habitat and food source for Elimia tenera, the freshwater snail. When the snails died, their shells sank to the bottom of the lake. The snails were so prolific that entire lenses of sediment were composed almost entirely of their shells.

After these layers were buried, groundwater moved through the sediments. Small amounts of microcrystalline silica that were dissolved in the groundwater began to precipitate, possibly in the form of a gel, within the cavities of the snail shells and the empty spaces between them. Over time, the entire mass of fossils was silicified, forming the brown fossiliferous agate (also known as chalcedony) that we know today as Turritella agate.”

References:

FTGVE102_Nov15_a

Fossil turritella agate & garnet earrings (Item FTG2)

http://geology.com/gemstones/turritella/
https://en.wikipedia.org/wiki/Elimia_tenera