Which is the hardest precious stone after diamond?

The ruby and sapphire tie for second-hardest precious stone with a hardness of 9.0 as measured on Mohs scale. (There are many minerals with the same hardness number, and corundum is simply the one selected as the test standard for this hardness.) both rubies and sapphires are made from the mineral corundum.

Of naturally-occurring gemstones, only diamond and moissanite are harder; only diamond is a precious stone.

https://en.wikipedia.org/wiki/Ruby

Note, to answer this question you must understand what a "precious stone" is:

There are only four true precious stones - diamond, ruby, emerald and sapphire. "All other gemstones are known as semi-precious and should not be called precious, although you'll often see them mistakenly called so." This distinctly narrows the field of choices from which to find an answer to the question posed!

The correct answer is Moissanite.

The structure of moissanite is one of its greatest properties. Similar to the diamond structure, moissanite’s structure gives it great strength, making it useful for testing applications and microelectronics. The crystalline structure is held together with strong covalent bonding that gives moissanite its strength along with other properties that rival diamond. Moissanite has little to no anisotropies occurring with in the crystal structure, thus giving it the ability to withstand high pressures and temperatures. On the Mohs scale of hardness, moissanite is graded at 9.25, second in strength only to diamond. Moissanite is harder than rubies and sapphires which come in at a hardness of 9, and significantly harder than cubic zirconia, which is a brittle material and takes damage relatively easily.

https://sites.google.com/site/aladawy/moissanite
https://en.wikipedia.org/wiki/Moissanite

Just to clarify -- Moissanite should be considered a jewel and not a gemstone:

Q: Why do you call it (moissanite) "jewel" rather than a gemstone/stone?
A: Terms such as gem/gemstone/stone should be avoided as their use is subject to Federal Trade Commission and Industry regulation to prevent consumer confusion.

http://www.4facets.com/moissanite.html
https://en.wikipedia.org/wiki/List_of_gemstones

Moissanite. Also rarer than diamonds.
"On the Mohs scale of hardness, moissanite is graded at 9.25, second in strength only to diamond. Moissanite is harder than rubies and sapphires which come in at a hardness of 9,..."
https://en.wikipedia.org/wiki/Moissanite

"...ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire."
https://en.wikipedia.org/wiki/Gemstone

(Remember hi-fi needles? They came in diamond, sapphire, or metal.)

This interesting site tells of some new synthetic materials that are harder than diamonds!

Diamond is believed to be the hardest naturally occurring material. New materials have been developed and synthesized which are even harder, so that nowadays diamond is not the hardest substance known to man.
What's Harder? Fullerites, or Polymerized Single Walled NanoTubules (P-SWNT)

Beta Carbon Nitride (?-C3N4) --- This is currently classed by some as a theoretical material, although in 2000, Northwestern University, Illinois claimed to have synthesized minute particles of it ...

Ultrahard Fullerite C60 --- The hardest substance known today is ultrahard fullerite, C60, which has an estimated hardness 1.17-1.52 times that of diamond. However, this material is currently available only in microscopic quantities: its scratch hardness had to be measured under the atomic force microscope.

http://24carat.co.uk/harderthandiamondframe.html

The reason moissanite is called a jewel instead of a gem is because it's made, not mined. It's synthetic. Naturally-occurring moissanite is so very rare it cannot be used by the jewelry industry.
http://www.moissanite.com/learn_more.cfm#a1
http://www.brilliantearth.com/news/moissanite-vs-diamond/

I'm now concluding it doesn't qualify as a "precious stone."

A good history of the term "precious gems" and the fact that there are only 4:

http://www.gemselect.com/other-info/precious-gems.php

http://kamayojewelry.com/gemstone-colors/precious-stones-names/

So indeed, it does appear that the answer to this question is technically "Ruby" because the word "precious" is used.

Using the term precious gemstone, makes a corundum the hardest gem on the MOHs scale after a diamond. That means both rubies and sapphires share second place since both are the same mineral.

https://en.m.wikipedia.org/wiki/Corundum

Nice find mira... I have updated Loonys top response as it was closest.

Padparadscha is another precious gemstone from the Corundum family of minerals. It has an orange color and a hardness of 9.0 on the mohs scale. So, of the Corundum minerals used as precious gemstones there is the red ruby, the blue or green sapphire and the orange (and very rare so it is less well known) padparadscha. .................. https://en.wikipedia.org/wiki/Sapphire#Padparadscha

The gemopedia (a decent scientific table) entry for Padparadscha at Jewelry TV can be seen here: ......................... http://www.jtv.com/library/padparadscha-sapphire-facts.html