Do you look up information about stones but don’t always understand what some of the terms mean? Specific Gravity? Streak? Moh’s Hardness Scale???
That’s what this weeks blog will try to answer. Don’t feel bad if you don’t know/understand what these terms mean for they’re not commonly used by the layperson. Geologists developed means to describe stones as a method for identification and classification. Minerals and rocks have different means of identification; minerals by things such as color, luster, and hardness, while rocks are often classified based on mineral composition. This article will focus on mineral identification, since that is what most jewelry is made out of (there will be a later article about rocks and their identification).
One of the first ways most everyone tries to identify stones/minerals is by color. Color can help narrow it down, but cannot necessarily be the only thing relied upon for identification. Looking at the first image, you see 6 different rocks pictured. Have an idea what they may be? Do you think they’re the same, or are they all different? The answer is actually both – the 3 samples on the left side are all calcite but in different colors, the 3 on the right side are all shades of green, but are different minerals: chrysocolla (in back), malachite (middle), and prenhite (front). Also notice how the prehenite and green calcite look remarkably close in color – a great example of why color cannot always be relied upon for identification. However it can help – for instance Malachite only occurs as a green mineral.
The ability for light to pass through a mineral is known as its transparency, and minerals are divided into transparent (light can pass through easily; e.g. quartz), translucent (some light is transmissible but you can’t see through it; e.g. chalcedony), or opaque (no light can pass; e.g. malachite) minerals. This is obviously not full proof either, as many minerals do not form perfectly, and thus may appear cloudy. The three samples of calcite in the first picture are a great example; while calcite is considered to be transparent, these samples are not.
Luster is how a mineral reflects light off of its surface. Metallic minerals such as pyrite are said to have a metallic luster, while nonmetallic minerals are usually described as having a vitreous/glassy (e.g. quartz), resinous (e.g. amber), pearly (e.g. gypsum), silky (e.g. muscovite) or dull/earthy (e.g. chert) luster.
Tiger eye (left), labradorite (back), moonstone (right)
Sheen refers to the reflection of light within a mineral caused by the internal structure. Hence not all mineral exhibit the properties to create a sheen effect. The cat’s eye effect known as Chatoyancy, is an example of sheen caused by the reflection of light on fiberous minerals; Tiger eye is a perfect example of this. Star sapphires and rubies are actually exhibiting chatoyancy that is occurring in a star pattern, or asterism. Iridescence is seen in labradorite, while Adularescence is the sheen visibile in opal and moonstone.
Isn’t a common method for identification but can be used – besides that its pretty cool too! Luminescence occurs when a mineral is held under ultraviolet light and the mineral glows (fluorescence if only while under the light, phosphorescence if it continues to glow when the UV light is turned off). This does not occur in all minerals, thus it can be used to some extent for identification. Common minerals that exhibit luminescence include fluorite, calcite, and opal.
Different minerals create different streak colors. From left to right: chrysocolla, malachite, galena, specular hematite, azurite
Streak is the color a mineral is in powder form. This is usually tested by scratching it on a white porcelain streak plate. Each mineral has a unique streak color, regardless of what color it may appear as. For instance quartz comes in many colors; however it always has a white streak (i.e. you see no color on the streak plate). Some minerals may have a streak that is the same color they are (e.g. galena is silver in color and has a silver streak), while others may have a different streak color (e.g. gray hematite will streak red).
(This is not recommended for good specimens or beads - use beads you don’t mind potentially getting scratched!) Hardness is just that – how hard a mineral is and is tested by using materials with a known hardness. The Mohs hardness scale was established with a scale ranging from 1 (softest) to 10 (hardest); each mineral has a known hardness number assigned to it. Those minerals with a higher hardness number will scratch those with a lower number.
Mohs Hardness Scale
9. Corundum (ruby)
Items with a known hardness can be used for testing, including your fingernail (2.5), coin such as a penny (3.5), glass (5.5), steel knife (6.5), and emery cloth (8.5).
Thus in the third picture you can see a piece of fluorite on the left, and rose quartz on the right; on the glass plate behind the samples, you can see a scratch on the edge that was made with the quartz next to it, but the fluorite was also drawn against the glass plate and left no scratch. In the next picture, you can see where the Flourite was able to be scratched by a nail, yet the Quartz could not be scratched by it. Both however are hard enough to scratch the penny.
These next few are not necessarily as helpful as the first, because they would either require you breaking your rocks/beads, or require very good specimens showing these features clearly. However I have included them since they are usually mentioned in any guide that gives specific properties about different minerals.
From Left to Right: Various Quartzes (Rutilated Quartz, Double Terminated Quartz, Rose Quartz), Malachite (all three in the middle), and Calcite (all three on the right).
This refers to how the crystals form, or their shape. It can vary from small crystals to large amorphous masses. Some of the common terms to describe habit include:
Tabular – generally flat with broad faces
Reniform – generally rounded like a kidney
Fibrous – thin, thread-like prismatic crystals
Botryoidal – bumpy or like a bunch of grapes
Acicular – many needle-like crystals usually in a radiating pattern
Massive – no distinct shape
Dendritic – tree-like branching shapes
Columnar – formed like columns
Stellate – star-like formation
In the above image, Malachite in the middle shows off its most common form in the middle, botryoidal, but it can also form as acicular crystals shown at the top. Once cut and polished however, the habit is no longer determinable (bottom).
Garnet, Calcite, and Muscovite
Cleavage & Fracture
Cleavage is where bonds are weak in a mineral and break along a plane. Sometimes when a mineral is broken, it will break along these cleavage planes. Muscovite for example, has perfect cleavage along one plane; it breaks along its cleavage planes very readily and is easily seen in most specimens. Calcite has three cleavage planes, and Garnet has none.
Fracture is the pattern that sometimes results when a mineral is broken – not very usuable, though perhaps most recognized is the conchoidal fracture that Quartz or Obsidian has.
On the left is Galena, Anhydrite on the right.
Specific Gravity is the density of a mineral. A hard one to use in some respects, as you would have to calculate this out for your beads/specimen. However in some instances it can be useful – for instance when trying to tell Galena apart from another gray, metallic mineral (of which there are many!) since it is very heavy. The above picture shows two different samples approximately the same size. Yet notice that Galena weighs about twice that as the Anhydrite sample!
Crystal system refers to how the mineral’s internal molecular structure is arranged. All minerals fall into one of six categories: Cubic (Pyrite, Garnet), Tetragonal (Rutile, Chalcopyrite), Hexagonal (Beryl/Emerald, Hematite), Orthorhombic (Sulfur, Olivine), Monoclinic (Mica, Gypsum), or Triclinic (Amazonite, Turquoise). Some also include Trigonal/Rhombohedral (Calcite, Quartz) which is very similar to the Hexagonal system thus included with it. This is really only useful for perfect specimens that may indicate their crystal sytem.
And that's it! Next weekend will continue with the regular stone of the week series!