Learn About Gem Stones and Minerals including mineral habit, lustre, Mohs scale of hardness, and more.
Here are some basics about mineral and gem stones including habit, lustre, Mohs scale of hardness, transparency, and weight.
Mineral habit is the shape of how a mineral tends to form. Some types of mineral habit are columnar, blocky, acicular (needle-like), granular, lamellar or foliated (sheets easily split apart), botryoidal (like a bunch of grapes), fibrous, radiating, or dendritic (like a fern). You can see a more extensive list of mineral habit types here.
A mineral's lustre can help identify it. Lustre is how the mineral reflects light. It can be shiny, vitreous, metallic, dull, waxy, pearly or others. Two of the most common lustres are vitreous and metallic. Vitreous lustre is glassy, like a quartz crystal or a water glass. Metallic lustre is metallic like a metal wire.
Some other lustres that are used are:
Adamantine - has a high degree of sparkle like a diamond
Earthy or Dull - does not reflect much light
Pearly - similar to a pearl in the way it reflects light
Resinous - similar to resin in the way it reflects light
Silky - similar to silk in the way it reflect light
There are many more which you can see here.
Mohs Scale - Mineral Hardness
The Mohs scale of mineral hardness was devised by Frederich Mohs, a German mineralogist, in 1812. To devise the scale, he selected ten minerals as a basis because they were common or readily available, thus the scale is not linear, but is a bit arbitrary. The Mohs scale is still the most common scale of mineral hardness in use today.
A scratch test will determine a mineral's hardness by comparing it to the mineral types on the Mohs scale. If an item scratches the mineral type, it is harder than that mineral. If it does not scratch that mineral type, it is softer than that mineral. Half ratings (e.g., 5.5) come from when a mineral does scratch one mineral but does not scratch the next hardest mineral on the Mohs scale.
Mohs Rating - Comparative Mineral Type - Common Item Equivalents & Notes
1- Talc - Talcum powder,
2 - Gypsum - Plaster of Paris, Fingernail (2.5)
3 - Calcite - Copper Penny
4 - Fluorite - Iron nail.Fluorite
5 - Apatite - Window Glass (5.5), Ordinary Pen Knife Blade
6 - Orthoclase - Steel File (6.5)
7 - Quartz - Good Steel Knife Blade
8 - Topaz - Sandpaper. Emerald and aquamarine are varieties of beryl with a hardness of 8
9 - Corundum - Sapphire and Ruby are types of corundum. They are two to five times as hard as topaz.
10 - Diamond - Four or more times as hard as corundum
If you are looking for information about a particular stone's hardness, mindat.org is an excellent resource.
Transparency describes how well light passes through a mineral . There are three basic degrees of transparency: transparent, translucent, and opaque. You can see objects through a transparent mineral, like you can see through a clear quartz crystal. Translucent minerals: You can see light, but no objects through the mineral. And with an opaque mineral, you can't see anything through it. Many minerals range from transparent to translucent or translucent to opaque. Calcite, for instance, can be translucent or opaque. Some minerals that are naturally translucent become opaque with weathering. Inclusions will affect the transparency of a stone.
Stones are weighed by carats, ounces, kilos, etc. Here are conversion tables for carats, ounces, grams, pounds and kilos.
1 carat (ct) = 1/5 gram (g)
28.3 grams (g)= 1 dry ounce (oz)
16 dry ounces (oz) = 1 dry pound (lb) = 454 grams (g)
1 kilo = 1000 grams (g) = 2.2 pounds (lbs) = 5000 carats.(cts)
By the way, a carat (ct) is a measure of weight, a karat (K or kt) is a measure of the quantity of gold in an alloy like 14K gold, and a carrot is Bugs Bunny's favorite treat.