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Luster: What does the surface of the mineral look like? It could be metallic or non-metallic. It either shines [or has a dull shine] like you expect in metals. An example of this is aluminum. The mineral might be non-metallic. There are lots of words to describe the surface shine of a non-metallic mineral. Some are: waxy [like a candle], dull [is not shiny], or pearly [shimmery or iridescent]. There are a lot of other words that we included in the actual mineral descriptions and we explained each one. Hardness: The Mohs Scale of Hardness is used to explain how hard a mineral is. Hardness ranges from 1 [soft] to 10 [hard]. Since a mineral can only be scratched by something that is harder than it is, its place on the Mohs Scale is usually explained with a description of the thing that will scratch it. A mineral that is higher in number on the scale, will scratch anything that has a lower number. For example, Topaz that is an 8 on the scale will scratch Talc that is a 1. On the other hand, Topaz won't scratch a diamond that is a number 10. Gemstones are always harder minerals. Even though there are other pretty minerals, they are not used as gemstones unless they are hard. Gemstones cost a lot of money and if they were soft, they would break easier. The gemstones couldn't be used for jewelry if they were soft. On the chart below, the gemstones are higher numbers since they are harder. scratch.wmv [1098 KB] To download: Right click on the link above and highlight "Save target as". MOHS HARDNESS SCALE Mineral Number What scratches it Some Minerals and Gemstones in each group 1 Fingernail [soft] Talc, Tin 2 Fingernail [soft] Asbestos, Biotite, Galena/Lead, Graphite, Gypsum, Halite/Salt, Lepidolite, Muscovite, Sulfur, Zinc 3 Penny Calcite, Chalcopyrite, Copper, Gold, Silver 4 Steel nail [medium] Nickel, Platinum, Sphalerite, Titanium 5 Steel nail [medium] Turquoise 6 Scratches glass [hard] Hematite, Magnetite, Opal, Pyrite, Uraninite 7 Scratches glass [hard] Amethyst, Aquamarine, Garnet, Olivine/Peridot, Quartz 8 Scratches glass [hard] Alexandrite, Emerald, Topaz 9 Scratches glass [hard] Ruby, Sapphire 10 Scratches almost everything [hardest] Diamond The hardness scale is not a normal scale with even changes from one number to another. It looks more like the diagram to the right. You can see that the numbers from one to seven are evenly spaced apart. This means that it is a gradual rise in hardness from one to two, from three to four, and so on. But when you get to the hardest part of the scale, the hardness increases a lot. This means that there is a big difference in the hardness of a diamond, for example, that is a 10 on the scale and a mineral that is nine. If you didn't see a diagram like the one on the right, you wouldn't know that there were such big differences between the numbers. Color: Mineralogists know what colors the minerals usually are. An example of this is the diamond. After studying diamonds for years and years, scientists can say, from experience, that diamonds are usually colorless, yellow, or grey. They also know that they can be other colors, too. Color is only one of the characteristics that they use to identify a mineral. Because there are differences in minerals [like colors], mineralogists don’t depend on just one of the characteristics to figure out which mineral it is. They use them all. Streak: This is the color of the dust that is left after you grind up a mineral. In school, we usually use something called a streak plate because we can’t just pound all the minerals into dust to test streak. Instead, we rub the mineral on the ceramic plate which has a hardness of 7. Any mineral that is softer than 7 will leave a little dust on the plate when it is rubbed on it. The color of the dust is a mineral characteristic. Sometimes the color of the dust is different than the color of the mineral. Cleavage: Cleavage is how a mineral splits. Some minerals split evenly in one direction into sheets, like Mica. Other minerals always split in three or four directions. The mineral does the same split every time. If you had 2000 chunks of Mica, every one would split in one direction into thin sheets. Because it happens every time, it is a way to identify a mineral. Fracture: When a mineral shatters, it doesn’t always break evenly like cleavage. It fractures into different shapes depending on what mineral it is. Some have jagged edges, some have smooth ones, and some break into grains or uneven chunks. Mineralogists expect certain minerals to fracture into jagged pieces because they always do. Magnetism: Mineralogists will check to see if the mineral is magnetic or not. If it is attracted to a magnet, it is magnetic. If it is not attracted to a magnet, it isn't magnetic. This is another way to identify a mineral. Specific Gravity: This is the weight of the mineral compared to the weight of water. This tells the scientist how heavy a mineral is. For example: platinum is very, very heavy. Mineralogists are able to narrow down what the mineral is based on its weight. Metals are heavier than regular minerals. Specific Gravity Density Minerals and Gemstones Light 1 - 2 Opal Medium 2 - 3 Amethyst, Aquamarine, Asbestos, Calcite, Emerald, Graphite, Gypsum, Halite/Salt, Muscovite, Quartz, Sulfur, Talc, Turquoise Medium/Heavy 3 - 4 Alexandrite, Biotite, Garnet, Lepidolite, Olivine/Peridot, Ruby, Sapphire, Sphalerite, Topaz Heavy 4 - 6 Chalcopyrite, Hematite, Magnetite, Pyrite, Titanium Heavier 6 - 10+ Copper, Diamond, Galena/Lead, Nickel, Silver, Tin, Zinc Heaviest 15 - 19 Gold, Platinum, Uraninite Crystal system: There are lots of different crystal formations. A mineral always has the same one. With each mineral, we named the crystal system with its proper name and then our team artist drew a picture of what the crystal looks like. We felt this would be easier to understand than axes, prisms, and symmetry. Transparency: Most minerals are transparent, translucent, or opaque. Transparent means that light can shine straight through it. An easier description is that you can see right through it like you do with window glass. Translucent means that light can shine through it but its path is changed when it exits. An easier description is that it is blurry to look through. Opaque means that light won't go through it at all. Our easier description is that you can't see through it at all.