It is the third-hardest mineral, after diamond and moissanite. It serves as the index mineral for a hardness of nine on the Mohs Hardness Scale….

Corundum in its pure state is colourless, but the presence of small amounts of impurities can impart a broad range of hues to the mineral. Ruby owes its red colour to chromium, sapphire its blue shades to the presence of iron and titanium; most corundum contains nearly 1 percent iron oxide.

Corundum is an aluminum oxide that commonly forms hexagonal barrel-shaped prisms that taper at both ends or as thin tabular hexagonal plates. It has a hardness of 9 on the Mohs scale, making it one of the most durable commercial gemstones. It has no dominant cleavage and fractures in a conchoidal manner.

Corundum is an aluminum oxide mineral of the oxides and hydroxides group, with structural formula Al2O3. The structure consists essentially of a dense arrangement of oxygen ions in hexagonal closest-packing with Al3+ ions in two-thirds of the available octahedral sites.

For example, no matter what color of corundum — sapphire or ruby — you test, it should always leave a white streak. However, if the allochromatic gem has received additional coloring agents, such as a dye, it may leave a colored streak. You can test this by rubbing a piece against a ceramic tile.

Rubies and sapphires are different colored varieties of the mineral corundum. Now corundum is a fairly common mineral. Gem-quality corundum is quite a bit rarer, consisting of only a percent or so of all the corundum found. The most common of the gem grade corundum is blue sapphire.

The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.

Diamond is the hardest mineral; no other mineral can scratch a diamond. Quartz is a 7. It can be scratched by topaz, corundum, and diamond. Quartz will scratch minerals that have a lower number on the scale….Mohs Hardness Scale.

As an example, you can scratch steel with a diamond, but you can easily shatter a diamond with a hammer. The diamond is hard, the hammer is strong. Hit steel with a hammer of any material and it just absorbs the blow by shifting the ions sideways instead of shattering.

Diamond is insoluble in water. Every atom in a diamond is bonded to its neighbours by four strong covalent bonds, leaving no free electrons and no ions .

Hydrofluoric acid (HF): A weak acid, meaning it doesn’t fully dissociate into its ions in water, but it’s probably the most dangerous acid in this list because it’s the one you’re most likely to encounter.