Page 14 - June SRG

14|June 2020
White Granite, White Quartzite, and White Marble: How Do You Tell Them Apart?
Slippery rock Gazette
One of the favorite parts of my job as a roving geolo- gist is to share not just an
appreciation for natural stone, but a deeper understanding of it. With a little guidance and practice, anyone can learn to recognize the properties and aesthetics of various types of stone. No matter a slab’s label, country of origin, or price tag, some basic geologic savvy can help us understand it better.
The process of identifying stones gets most confusing when they look alike, and nowhere is this more vexing than with white stones, which are popular, fre- quently mislabeled, and often misunderstood. But fear not. Even similar-looking stones can be sorted out by using a few basic guidelines.
Read on, as we step through the process of differentiating light-colored granites, quartzites, and marbles.
Granite Has a Distinct Look, Compared to Quartzite and Marble
Granite is the most common type of natural stone in the trade and comes in a satisfyingly large range of colors and patterns. Despite their diverse aesthetics, granites have common elements that make them recognizable. Below are some rules of thumb. Please note that I’m referring to the general industry classification of granite, rather than the nar- rower geologists’ definition.
Karin Kirk
usenaturalstone.com
Diagrams ® Karin Kirk.
Photos by Karin Kirk and courtesy
• Light-colored granites have flecks or blocks of varying col- ors. White granites are rarely a homogeneous, even-toned color. Most light-colored granites have more than one color in them, and it’s rare to have a white granite without any darker minerals at all. • Individual minerals are visi- ble. Look for blocky crystals of feldspar, glassy areas of quartz, and a smattering of darker col- ored minerals. (Learn more the different minerals in stone at www.slipperyrockgazette .net/archives/Definitive-Guide-to -Quartzite/ )
• Sometimes granite has giant minerals! If the slab has minerals larger than couple of inches, then you’ve got a pegmatite. That’s special type of granite with su- per-sized crystals. Patagonia is one of the more dramatic ex- amples of a granite pegmatite, and Tourmaline, Alpine, Alaska White, and Delicatus are other examples.
• If it contains garnet, it’s gran- ulite or gneiss. Garnets are small, round minerals that are dark pink, burgundy, or reddish brown in color. Their presence is an instant giveaway that you’re looking at metamorphic variations of gran- ite. Gneiss has stripes or bands of lighter and darker minerals, while granulite tends to have few or
no stripes and is generally light- colored overall. There are many white granulites on the mar- ket, including Bianco Romano, Colonial White, or Giallo Cream. Viscount White is an example of gneiss.
• Mica is common in gran- ite. Mica is present in small amounts in granite, and it makes an appearance as glittery minerals that can be silver, gold, bronze, or metallic black. If a stone is mostly made of mica, then it’s schist.
• Granite can have quiet pat- terns or vivid movement. Leave it to Mother Nature to create waves of color, veins of con- trasting minerals, and all sorts of other interesting effects. That’s a big part of the appeal of a natural stone.
Marble and Quartzite Look Different from Granite, but Similar to Each Other
Marble and quartzite look alike in several ways:
• They tend to be mostly light-
colored: white, light grey, cream. Darker colors are also possible.
• Marble and quartzite are usually fine-grained, overall; you can’t put your finger on an individual mineral grain.
• They often have layers or bands of contrasting colors. The layering can be straight, wavy, or chaotic.
• Slabs can have quiet patterns or vivid movement.
• If a slab has fossils, then
it’s limestone, not marble. Limestone has a chalky or matte finish compared to either marble or quartzite.
Please turn to page 15
This granite features black bands of mica – beautiful, but because mica is softer than the surrounding crys- tals, it can be challenging to fabricate.
White stones are deservedly popular. Geology skills can help tell them apart and guide you toward a decision about which one is right for your customer.
A typical white granite contains crystals of different colors. Below: Even an all-white granite still contains various blocky shapes and shades of white. This granite contains mostly feld- spar with smaller, v-shaped quartz crystals. Geologists call this “graphic granite” because it looks like a stone tablet of cune- iform writing.