Page 14 - May Slippery Rock Gazette

14|May 2020
Green Stones
From Earth’s Mantle to the Fireplace Mantel: Tracing the Origins of Green Stones
Slippery rock Gazette
with minor amounts of chlorite that tint the white stone light green. Dark green stones that are called marble are more likely ser- pentine. Either way, the acid test should tell you the difference.
Soapstone
The world of natural stone could be divided into two categories: Those who cannot live without soapstone, and everyone else. Soapstone has a unique set of properties that set it apart from other stones and garner a devoted following. Acids, alkalines, heat, or cold don’t faze soapstone, as it shrugs off liquids, stains, and tem- perature extremes.
Alas, soapstone’s Achilles heel is that it’s relatively soft. Daily wear and tear on a kitchen coun- tertop will take a visible toll on soapstone. That said, scratches can be sanded out or made less visi- ble with a coating of oil. Another option is to simply use soapstone in easier-wearing situations, like a buffet, bar, or bathroom. Soapstone’s ability to absorb and re-radiate heat makes it an unri- valed material for wood stoves, fireplaces, or mantels.
Soapstone can be deep green, grey, or black. It’s usually dis- sected by white veins in easygoing patterns. Not only is the stone it- self soft, the whole aesthetic is too, with subtle color variations and a muted luster. This stone is equally suited for a Vermont farmhouse or a contemporary loft, lending time- less appeal to any style.
Talc is the primary ingredi- ent in soapstone, but it’s not the only ingredient, and that’s why the hardness of soapstone varies. Talc is the softest of all minerals.
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Appalachian Green Danby marble
Photo Courtesy Vermont Quarries Corp.
The natural world wraps us in green; it’s an ever-pres- ent color of nature. But it’s not a common color among natural stones. That’s a shame, because green is expressive, easy on the eyes, and fun to design with. But fear not, Mother Nature didn’t totally forget green in her palette of stone colors. Some stones, like serpentine, are known for their verdant hues. While others, like soapstone and granite, have green variants that stand out from the earth-tone crowd. Let’s explore some green stones and illuminate their properties, min- erals, and geologic origins.
Serpentine
The green, mottled pattern and waxy feel of serpentine in- spired its name - a reference to snake skin. Serpentine can be a vibrant bright green, or a deep forest-green. It’s fine-grained and smooth, and it polishes to a satiny luster. Accents of brown, red, and white can liven up the stone with vivid movement, or it can have a serene aesthetic with gentle swirls of green and ivory.
While the distinctive look of serpentine makes it easy to spot, it’s one of those stones that can get a bit confused within the in- dustry, leading to some conflict- ing information. Often serpentine is labeled as green marble, and sometimes it’s classified as “hard soapstone.” Alas, it’s neither. Unfortunately, serpentine can be a little tricky to identify because its hardness, color, and pattern can vary. That’s because serpentine isn’t one exact rock; it’s a family
What is This Green Stone?
Karin Kirk
usenaturalstone.com
Cutting face of Verde Antique quarry in Vermont. Verde Antique is a hard serpentine.
of stones that all form in a similar way, but can have slightly differ- ent minerals in them.
If you were to do a little Googling about serpentine, you’d discover phrases like, “Serpentinites form as a result of serpentinization.” Helpful, eh?
Thankfully, we can clarify that explanation. Serpentine (geolog- ically known as serpentinite, but we’ll keep things simple here) is a metamorphic rock, which means it was first some other kind of rock and then experienced a change in conditions. The precur- sor to serpentine is an iron-rich magma that forms way under- neath the ocean floor. In fact, this magma’s source is so deep that it comes from Earth’s mantle, many
Diagram by Karin Kirk
miles down in Earth’s interior. This deep, dense magma gives rise to the bright green mineral olivine, which is also known as peridot, August’s birthstone.
When this iron-rich, green stone mingles with hot seawater, new types of minerals are formed. This process is common, but it happens so deeply within the Earth’s crust that it’s rare to see it up here on the surface. But thanks to plate tectonics, deep rocks sometimes get shoved upwards, making ge- ologists giddy for a glimpse of what’s happening far below our boots.
Green soapstone is one of the softer types of green stone. Photo Courtesy MSI
If the stone has white veins, it’s a good idea to do another acid test to see if the white parts of the stone etch when exposed to acid. Sometimes serpentine has white veins of calcite, but otherwise it should not be bothered by house- hold acids.
Serpentine tiles can have a tendency to absorb water when “wet set” with water-based mor- tar, which causes warping of the
Because it’s made of a range of minerals, serpentine also has a range of hardness, between three and six on the Mohs scale. That means it’s harder than marble but softer than granite. It’s a good idea to do a scratch test to inves- tigate the hardness of the stone. This is best done with the tip of a sturdy pocketknife, an awl, or a steel nail. Make scratches in dif- ferent parts of the slab to check the hardness of different colored areas, since those are made of dif- ferent minerals.
Even though serpentine and marble have similar hardness, it’s easy to tell them apart. Marble will always become etched from contact with acids. So put a drib- ble of vinegar on the stone, let it sit for a few minutes, wipe off the vinegar, and inspect the stone for a change in coloration or a change in luster. If the stone is affected by acid, it’s marble. If not, it’s serpentine.
Mohs hardness scale, and some simple tests to help identity your green stone
Large format flooring tiles: Is it marble or serpentine?
stone. Proper installation methods can avert this problem.
Well-known serpentines are Rainforest Green from India, Verde Antique from Vermont, and so-called Connemara ‘Marble’ from Ireland.
Marble
True, green marbles are Appalachian Green Danby and Cipollino marble from Greece. Cipollino has thin stripes of green and white — evocative of the layers of an onion — earning it the stonecutter’s nickname, Cipollino. These green marbles are primarily made from calcite
Beautiful, striped Cipollino marble from Greece
Stone can be scratched with a copper penny (HARDNESS LESSS THAN 3.5)
Intermediate Hardness (HARDER THAN A PENNY, SOFTER THAN GLASS)
Does the stone etch with vinegar or lemon juice?
YES NO
Marble
Serpentine Soapstone HARDNESS RANGES 3-6
The stone scratches glass easily
— OR —
A nail / tip of knife blade makes little or no scratch on the stone (HARDNESS +6)
HARDNESS 3-3.5
HARDNESS RANGES 1~3.5
Granite Quartzite
12345678 SOFTER MOHS HARDNESS SCALE HARDER