A Handy Guide to the Ingredients of Your Favorite Natural Stone

Karin Kirk

usenaturalstone.com

Many thanks to Slabworks of Montana and Montana Tile and Stone for allowing me to explore and photograph their beautiful stone slabs.

Minerals are the components of all natural stones. The color of every natural stone, whether it’s jet black, glittery silver, or a kaleidoscope of Technicolor – comes from the individual minerals.

Minerals also give each stone its personality. Is it flashy, or subtle? Is it a uniform color, or a melee of diverse ingredients? 

And, of course, minerals dictate the properties of a stone: Hard or soft; acid-resistant or acid-sensitive; flaky, chunky, or smooth.

Given that minerals determine so much about a stone, it warrants a look at some of the more common minerals, how to spot them, and what they tell you about a stone.

Before we dive in, one important point is that you can only see individual minerals in coarse-grained stones. A smooth stone with small grains, like Absolute Black or Pietra Grey, doesn’t reveal much about specific minerals because you can’t see them. But many popular stones have big crystals in all kinds of patterns and colors, inviting curiosity about just what those minerals are all about.  


Feldspar

Feldspar is the most abundant mineral in the Earth’s crust, but it’s far from mundane because it occurs in a huge range of colors and forms. When you look at a slab of typical granite, you’re looking at mostly feldspar. Igneous rocks like granite take shape as they solidify from liquid magma. You can think of magma as a ‘slushy’ drink. It’s a mixture of solid bits and liquid. Feldspar crystals are often the solid chunks within a body of slushy magma, and you can see the chunky texture of feldspar in some granite slabs.

In other cases, feldspar is altered by metamorphism, changing its shape from pushing, pulling, or shearing. In these cases, the blocky shapes of feldspar crystals can become more rounded. 

Above, left: In this slab of Sea Foam Green it’s plain to see that the feldspar crystals floated around in a liquid magma. Above, right: Feldspar often forms rectangular or blocky-shaped crystals. In this slab of Ocre Itabira there are two types of feldspar. One is silvery white and the other is light brown.

Above, left: In this slab of Sea Foam Green it’s plain to see that the feldspar crystals floated around in a liquid magma.

Above, right: Feldspar often forms rectangular or blocky-shaped crystals. In this slab of Ocre Itabira there are two types of feldspar. One is silvery white and the other is light brown.
Above, left: Blue feldspar — called Labradorite — in a slab of Volga Blue. Above, right: These feldspar grains lost their blocky shape when the stone got squeezed and stretched during metamorphism. The black, wavy layers are black mica.

Above, left: Blue feldspar — called Labradorite — in a slab of Volga Blue. 

Above, right: These feldspar grains lost their blocky shape when the stone got squeezed and stretched during metamorphism. The black, wavy layers are black mica.
All feldspar, all the time! This stone is 100% feldspar, arranged in a textbook-perfect example of ‘interlocking’ texture that shows how the crystals have grown tightly into each other. The different shades of grey are caused by the different orientations of the crystals, so they reflect light in different directions. This is leathered Antique Brown, which a geologist would call ‘anorthosite,’ and I would call ‘fabulous.’

All feldspar, all the time! This stone is 100% feldspar, arranged in a textbook-perfect example of ‘interlocking’ texture that shows how the crystals have grown tightly into each other. The different shades of grey are caused by the different orientations of the crystals, so they reflect light in different directions. This is leathered Antique Brown, which a geologist would call ‘anorthosite,’ and I would call ‘fabulous.’

Color:

Feldspar crystals can be white, black, and any shade of grey. They can also be pink, cream, brown, and sometimes green. Best yet, the variety of feldspar called labradorite is iridescent blue, and plays the starring role in Blue Pearl and Volga Blue.

Identifying Features:

Since feldspar can be nearly any color, using color to identify it won’t help at all. But it does have a few features that make it recognizable. 

  • Feldspar is not glossy and its luster is similar to porcelain when its not polished. 
  • It is always opaque, meaning, you can’t see ‘into’ the crystal at all. 
  • It’s more or less rectangular in shape.
  • The crystals break into naturally flat faces called cleavage planes. This is especially visible on a honed or leathered slab when you look at it from an angle. The flat faces will catch the light. This is also evident on the edge of a slab where you can see a crystal in three dimensions. 
  • You can sometimes see subtle stripes or grooves in a feldspar crystal.




Garnet

Garnet is January’s birthstone, and it’s a beautiful mineral. Garnet is dark-raspberry pink, maroon, or maroonish-brown. 

Above, left: Garnet’s dark pink color and round crystal shape make it easy to recognize. Above, right: These garnets come to life in the sunshine of the slab yard. The white mineral is quartz and the black is biotite mica.

Above, left: Garnet’s dark pink color and round crystal shape make it easy to recognize.

Above, right: These garnets come to life in the sunshine of the slab yard. The white mineral is quartz and the black is biotite mica.

Identifying features: 

  • The color is usually a total giveaway.
  • The crystal shape is usually round-ish. It often occurs in specks.
  • Garnet crystals have a glassy luster, and when viewed on the edge of a slab or in an un-cut rock, they are brilliantly sparkly.

Properties:

Garnet is 6.5 to 7.5 on Mohs scale and is often used as an abrasive. Large, translucent crystals of garnet are used for gemstones. 

Examples: Garnet is not a major ingredient in any stone, but its recognizable crystals are in River White, Colonial White, Dallas White, and St. Cecilia, among others. 

Superpower: People who like garnet are above average at math. Unfortunately, simply owning a garnetiferous stone will not, in and of itself, improve your math skills. 


Join us next month for Part 2 of Know your Minerals.


Karin Kirk is a geologist and science educator with over 20 years of experience and brings a different perspective to the stone industry. Karin is a regular contributor to usenaturalstone.com and the Slippery Rock Gazette. Contact her at karinkirk@gmail.com.