Top 10 Extremely Hard Gemstones

Gemstone hardness is the ability of one mineral to scratch another. Mohs' scale is used in jewelry to measure the hardness / durability of the gemstones.
Gemstone hardness is a major criterion for gem quality. The other one is perhaps the color and sometimes not very durable gemstones are valued for the color or appearance.

I made this list for two reasons:
1) to show that gems are highly valued for their hardness, although this isn't the only factor
2) to see whether people would choose a softer gem if they like the color (that's why I made this list votable)

Numbers next to the gem names show gems' hardness on the Mohs' scale, from 10 to 1, where:
10 - means the hardest
1 - means the softest (mineral talc best represents this group).

Extremely hard gems are those placed from 7.5 to 10 on the Mohs' scale of hardness.
The magic number 7 (Quartz hardness) in jewelry means: if a gem is softer than 7 on Mohs scale, extra precautions must be taken.
The items in this list have been selected by the author of the list for you to vote and comment on.
The Top Ten
1 Diamond - 10
2 Sapphire - 9

Sapphire and Ruby are almost identical gems and I put sapphire higher than ruby only because sapphire occurs in all colors except red, while ruby can be only red which limits your color choices if you don't like red.
Look, sapphire cannot be red because the red variety was already called ruby. But both Sapphire and Ruby are varieties of the same mineral - corundum.

3 Moissanite - 9.5

It sparkles more than diamond

4 Ruby - 9

Ruby is like the red variety of sapphire

5 Alexandrite - 8.5
6 Cymophane / Cat's Eye / Cat's Eye Chrysoberyl - 8.5
7 Taaffeite - 8 to 8.5
8 Musgravite - 8 to 8.5
9 Topaz - 8

This isn't the only topaz color (the image shows imperial topaz which is the highest valued topaz)

10 Spinel - 8
The Contenders
11 Chrysoberyl - 8.5 
12 Painite - 8

The rarest gemstone in the world.

13 Morganite - ‎7.5 to 8
14 Green Beryl - ‎7.5 to 8
15 Dumortierite Quartz - 7 to 8.5
16 Aquamarine - ‎7.5 to 8
17 Emerald - ‎7.5 to 8
18 Red Beryl (Bixbite) - ‎7.5 to 8
19 Tourmaline - 7.0 to 7.5 
20 Goshenite - ‎7.5 to 8

Used as a diamond alternative

21 Amethyst - 7.0

It exemplifies the magic number 7 (Quartz hardness) - Amethyst is a violet variety of quartz

22 Andalusite - ‎7.5
23 Hambergite - ‎7.5
24 Grandidierite - 7.0 to 7.5