What is emery?

Search: emery

Why: An article I am editing talks about an "emery cloth," and, y'know, emery boards exist.

Answer: It's a really hard rock! It's a mixture of minerals, including corundum, which - I think we all remember from last March - is the fancy science name for rubies and sapphires. Crushed or naturally eroded emery (known as black sand) is used as an abrasive and as a traction enhancer in asphalt and tarmac mixtures. It is mostly mined from the Greek island of Naxos.

Source: Wikipedia

The More You Know: The Mohs Scale of Mineral Hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It's based on 10 minerals that are readily available:

1. Talc (Absolute Hardness 1)
2. Gypsum (3)
3. Calcite (9)
4. Fluorite (21)
5. Apatite (48)
6. Orthoclase Feldspar (72)
7. Quartz (100)
8. Topaz (200)
9. Corundum (400)
10. Diamond (1800)

So: if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5. Since emery can be a mix of minerals, there is no assigned Mohs hardness. The hardness of corundum is 9 and that of some present spinel-group minerals is near 8, but the hardness of others, such as magnetite, is near 6.

Other interesting hardnesses:

• Pencil "lead" (graphite) - 1
• Fingernail - 2.2-2.5
• Copper penny - 3.2-3.5
• Pocketknife - 5.1
• Knife blade - 5.5
• Window glass plate - 5.5
• Steel file - 6.5
  
• Unglazed porcelain - 7
• Cubic zirconia - 8