There are several issues when designing a shield.
What is a shield? It is something that protects you against some incoming weapon. It can do that by deflecting the weapon, by absorbing it, or by reflecting it, or some combination of all three. What is the weapon? This is not specified, but presumably the weapon you have to fear is the one that is best at getting through the shield. So the shield and the weapon evolve together. This is particularly true of things like tank armour and armour piercing shells. A good shield ought to be light. I don't know if this extends to magic but I think it ought to.
The incoming weapon has momentum and energy. The best thing to do is to deflect it. That way the shield will only have to take some of the momentum and a small amount of the energy. If you can, have your shield at a small angle to the incoming weapon. If you can't anticipate where it is coming from, angle your armour so a weapon from most angles will glance off. That is the first job of plate armour.
This isn't easy with a flight of arrows. If you put the shield at a small angle to the incoming arrow, then it is hard to hide behind. On to the second option: absorb the energy. A hide shield is light and good against arrows - the arrow can penetrate the hide, but unless you are holding it against your body, you are still protected, and the shield with an arrow in it is still a shield, but your arm does not have to take the sudden shock when the arrow hits.
This is also true with tanks and armour-piercing shells. A HEAT shell can generate supersonic needle of molten metal, which can get through feet of metal plate. The defence is to have an irregular barrier out front that breaks the point of the needle, and turns it into a spray. There is then more material or void that allows the spray to cover an area, followed by something like ordinary armour plate with a high work to fracture that can absorb the energy.
A good shield is a composite. The front layer will deflect or blunt if it can, the middle region should spread the attack over a larger area, and the the final layer should hold together, and stop bits of flying shield hurting you. Each layer has separate material requirements.
Crystals typically have a very small work to fracture. Diamonds can be cleaved by a sharp edge. Nephrite is not a crystal in the ordinary sense; it has a fibrous structure like mineral asbestos, so it is effectively a composite like carbon fibre. This makes it less hopeless than other crystal materials. It can make a passable axe-head but it is not better than steel if you are making a shield. I do not know the origin of that factoid, but the quote I found has no figures or science. Wood would be better for that.
'Angels with shields of pure crystal' may look good in visionary writing, but like 'harps strung with gold' it will not work. Materials science replaces our vague word 'tough' with lots of separate properties, each of which may be critical at some point within a composite device. This was my subject. You can't just search for 'what is the toughest material?' and make a shield of that. But you asked, and that's the main thing.