Diamonds are NOT a girl's best friend - or so I tell my wife. And here is why.
All minerals are formed from atoms and compounds that take on a certain crystalline form due to the temperature and pressure conditions at the time of their formation. Once you take them from those formation temperatures and pressures and place them, say on the Earth's surface, the atoms tend to want to rearrange into a crystalline form that is stable at those new temperatures and pressures.
We see this happen when metamorphic rocks are formed. During this process some minerals change WITHOUT melting into different minerals such as clays in shale changing into micas in schists because of the change in temperature and pressure.
We also see this change when minerals are exposed at the Earth's surface, with micas weathering down to clays. In this case of course there is a chemical change, but a driving force is that the mineral is more stable in a different form at the Earth's surface and in those new conditions.
What about diamonds?
So lets look at diamonds. Diamond is one of the mineral forms of carbon. Diamonds form at extremely high temperatures deep in the Earth's mantle (some 140-190 kilometers below your local jewelry store). Here they grow over vast amounts of time and only make it to the upper Earth's crust during violent eruptions of magma that form kimberlite or lamproites pipes.
Now carbon forms other minerals as well. Graphite is a mineral of carbon that forms at much smaller temperatures and pressures than its diamond cousin.
So, I tell my wife, taking diamonds from such great temperatures and pressures and putting it on the Earth's surface of very low temperatures and pressures means the mineral is unstable (I think I use the term 'mega-unstable' to her... for effect). The carbon atoms would be far more stable is they were in the mineral form of graphite.
And so, I conclude, I would not want my beloved wife to be wearing such an unstable item! "See honey...diamonds are NOT a girl's best friend".
Now quartz on the other hand is very stable and forms easily at the Earth's surface. Maybe a ring with a nice piece of cut quartz would make a far better gift?
That's my story...and I am sticking to it!