There is literally so much content to talk about regarding the purpose, structure, feasibility and effectiveness of this idea that isn't related solely to the evolution of such!! But I'll just focus on the crystal and development side; alright, here 'goes.
What crystal could be used to create this sail?
As it turns out, the gemstones you've listed take on more the appearance of rock than crystal in their natural state, rather dull and usually suffocated by other minerals; they only get their inherent beauty from being cut and polished in specific ways, if they weren't, well, jewellery wouldn't be nearly as popular. They also will be unusable, these gem stones - sapphire and ruby and diamond - are all formed in extreme environments from being exposed to lots of heat and pressure, such amounts your creature wouldn't safely be able to create through natural biological processes.
So we look to alternate crystalline sources for inspiration, of which there are many; Fluorite, Calcite, Celestine, Barite, Adamite, Halite, Colemanite, Beryl structures, Aragonite, Armalcolite, all these minerals form crystals that (provided the correct conditions) can grow very large indeed; they also have the advantage of being relatively easy to form under "normal" environmental conditions - such as on the back of your creature.
"I'm not sure whether a singular large piece of crystal is better/less fragile or having multiple crystal pillars is better"
This is where some problems arise. Having a singular giant crystal on the back of your creature will not work, as it takes a great deal of time to build, per se, and will (if formed fast enough) be too thin and fragile to serve any purpose. This logically suggests that multitude of smaller pillars may work... 561
not really. They would still take an eon to grow to the size needed, from scratch.
But this got me thinking: as a child, who hasn't tried growing crystal in cups of water, using lots of string to give it more space to develop? This is effectively the system your creature will need to develop it's sail: some kind of large framework to grow the crystals on top of, giving it a larger surface area to bind to hence allowing it to grow faster from more locations. And what better material to use than a skeleton (calcium carbonate foam).
Coral is an ideal example of what I mean, able to expand outwards into the environment by building bone-like structures to grow upon; this gives them a very large surface area too, meaning lots of room for additional coral organisms to fix to, allowing it to grow to huge sizes.
Your creature will need to have a similar, coral-like structure protruding from its back (cartilage will do just fine) that forms the basic shape of your sail where crystals can grow. This leads us to the evolutionary part of your question:
How would it have evolved to grow crystals in this way?
The animal your new creature has evolved from would have also had a sail, or fin-like protrusion from its back, that by all means could have acted in the same way a Dimetrodon or stegosauruses is suspected to have (for thermo-regulation purposes/ attracting a mate).
It would have had to have lived in/near an environment that was rich in the elements required to produce these crystals (they being iron, copper, silicon, fluorine - all abundantly available), for instance the land surrounding VOLCANIC PLAINS! They are the ideal environments, readily life-sustaining and rich in mineral-saturate lakes/pools that's sediment and water contain these required trace elements.
All it would have needed to do is move through these mineralised waters on a regular basis; when the water evaporates off it's skin (the majority being on it's sail) the crystal forming elements are left behind; they react with the oxygen in the air and form the foundations for further crystal growth.
As time progressed and a thicker layer of oxidised copper/iron/fluorine/ gathered, the sail became encased in a thin crystal-armour; this was good for protecting the sail, but inevitably led to it's overall destruction (deteriorating the skin, causing it to die and leaving only the bare bones behind, which quickly became coated in the same crystal).
As generations went on, young were born without their sail-skin, yet retained bones in the more flexible form of cartilage; as the creature grew these main cartilage spines split and branched inwards on one another like coral, creating a sort of mesh framework that resembled it's previous sail. 671