Trying to identify what you picked up, or checking to make sure you got what you paid for. Either way, with some minor investigation and a web page or two, you should be able to identify most, but not all specimens. Some differences are so slight that a laboratory would be needed to tell the difference.
Any mineral has a number of physical characteristics that can help you to identify it. Once known, these can help to confirm an identity, or at least narrow it down by process of elimination. They can be broken down into colour, hardness, transparency, crystal system, habit, lustre and so forth. Here they are in more detail.
The Six Crystal Systems
All known mineral crystals grow in one of six crystal systems. There are many forms in each system, but they are defined by the relationship of imaginary axes to opposing crystal faces. A crystal is also symmetrical along an axis. This is all supposing the specimen has had ideal conditions of temperature and pressure and space to form properly. Many crystals have been formed and deformed by these processes so that the crystal system might not be apparent. Remember that crystalline structures are the same no matter the size of the particle, so a high powered magnifying glass will help with some specimens.
Three imaginary axes, all the same length(a1 = a2 = a3), and all at right angles to each other. Common forms are the cube, octahedron and dodecahedron. Examples include Pyrite, Garnet and Diamond.
Three imaginary axes, with the two horizontal axes of equal length, and the vertical axis either longer or shorter (a1 = a2 ≠ c), all at right angles to each other. Zircon and Rutile are examples.
Three imaginary axes, all of different lengths (a ≠ b ≠ c), all at right angles to one another. Examples include Topaz and Chrysoberyl.
Three imaginary axes, all different lengths (a ≠ b ≠ c), two of which are at right angles to each other. Epidote and Titanite are examples.
Three imaginary axes, none of the same length (a ≠ b ≠ c), and none are at right angles to each other. Albite and Feldspar form in this crystal system.
Four imaginary axes, three of the same length (a1 = a2 = a3), the fourth being longer or shorter. Angles on the a axes = 60 degrees with a 90 dgree angle with the c axis. Beryl is an example.
This one takes a bit of practice and is sometimes more useful in determining what your sample isn’t than what it is. If you can, try and find a crystal that is well formed and on it’s own as an example. To make it more interesting, there are also pseudomorph’s that take on the form of a different mineral species to keep you guessing.
Sometimes called sheen, this refers to the way light reflects off the surface off a mineral or crystal. The first category is Metallic (Gold, Silver, Hematite) and Non-Metallic
Non-Metallic is broken down into the following categories –
Adamantine – Bright and sparkly, like a Diamond.
Vitreous or Glassy – Like broken glass. Quartz, Topaz.
Resinous – Like resin. Amber is the best example.
Pearly – With an iridescent shimmer.
Silky – With fine parallel threads, almost like fabric. Fibrous is a courser version
Greasy – Slightly oily to the touch. Turquoise or Opal.
Dull or Earthy – The mineral reflect little light, like Howlite or Rhodonite. Earthy is courser.
The ability of a mineral to let light through. Broken down into 3 broad categories.
Transparent – Mineral lets most of the light through. Objects can be seen clearly though the specimen. For most gemstones, the more transparent, the rarer and more expensive. examples – Clear Quartz (rock crystal), some Calcite, cut Diamonds
Translucent – Mineral lets some, but not all light through. Objects may be seen, but not clearly. This has a broad range, from specimens that are nearly transparent (with some clouding or impurities), to specimens that appear opaque until a bright light is used and some light might be seen around the edges. examples – Agate’s, Selenite, Calcite
Opaque – Mineral let’s no light through. examples – Most metals, Jasper, Unakite, Howlite
Some minerals can be found in all the categories, so translucent to opaque, transparent to translucent, and even transparent to opaque are possible.
One of the easiest physical characteristic to use for identification, it can also be the most misleading. Some minerals, like Ruby, can always be identified by their colour. Others, such as Tourmaline and Calcite, may be found in a variety of colours.
Minerals can be allochromatic, or colourless in their pure form, where any colour is caused by impurities or other substances. Examples are Rock Crystal, with a touch of added Manganese, becoming Amethyst.
Minerals can also be idiochromatic, meaning having a colour of their own, not caused by impurities. Examples are Turquoise and Lapis Lazuli
A good indication of identity, based on Moh’s scale, which places minerals on a list, each of which can scratch the mineral below it on the scale. Diamond is at the top of the scale with 10. Talc is all the way at the bottom at 1.
10 – Diamond
9 – Corundum
8 – Topaz
7 – Quartz
6 – Feldspar – Knife blade – 6.5 – 7 – Unglazed Porcelain (Streak plate)
5 – Apatite – 5.5 – 6 – Glass
4 – Fluorite
3 – Calcite
2 – Gypsum – 2.5 – 3 – Fingernail
1 – Talc
If you have to do this test, try to find a small concealed area to make the scratch. You don’t want to deface your beauty finding out what it is. Also be aware that some minerals have varying hardness depending on the content. Some minerals, like Kyanite, have a different hardness based on if you scratch it with or against the grain. So look for a pure piece of the mineral without impurities.
Refers to the way a crystal splits or fractures when broken. This is caused by the atomic structure of the mineral.
If it splits along predictable lines, then this is further broken down into – none, poor, fair, good, perfect.
If it doesn’t cleave, then there are further categories – conchoidal, fibrous, hackly, uneven.
Seeing as you have to break the stone to test the theory, this physical property isn’t used often for identification.
This refers to the colour of the powder formed by the mineral. The easiest way to test this, is to rub your specimen on an unglazed porcelain tile (Streak plate).
This might yield some interesting results. All quartz ( Citrine, Smoky, Amethyst etc) is colourless. Hematite, which appears black, leaves a reddish brown streak.
Sometimes the levels of mineral contributing to the color of a specimen, is a very small amount, and doesn’t show up in the streak.
This refers to the form or shape that minerals are generally found in. Minerals often do not conform to the textbook form of their crystal systems by outward appearance. In tiny form, they follow the rules, but outwardly they grow in a bewildering array of forms.
The form of minerals found in their natural state is dependent on changes in temperature, pressure, the chemical solution it crystallizes from and available space to grow.
They are variously defined as – Acicular (needle like), Bladed, Fibrous, Cauliform (shaped like a cauliflower), Tabular, Prismatic, Dendritic (tree shaped), Reniform (kidney shaped), Lamellar (thin sheets) and Massive (big lumps with no discernible shape).
Normally most stones can be identified using colour, lustre, transparency and their crystal system. If you have identified those, and are still unsure, you can try and identify the streak, hardness and cleavage. Either way, don’t be surprised if you have a few stones that remain unidentified for a long time.