Gems

Reason that gemstones are used in spells as spell component is due to the specific kind of energy that they hold. Ruby, a gemstone commonly used in spells related to fire, has extremely good relation with those spells due to the power it contains. While some spells merely resonate with the gemstone to "borrow" their power as a spell focus, others consume them utterly to fuel the spell itself.

Gemstones are good containers for spells as well - unlike a scroll, which records magical incantation written by a mage parting a portion of their power and infusing it into the scroll (which occasionally use gemstones in their inks, of course), the gemstones that hold the spell tend to require much less magical knowledge once created - a powerful throw, or striking against a hard surface, can often cause the spell to occur.

The cut of the gemstone determines their mundane value, which also "expresses" the gemstone. A badly cut diamond, one that is off center and reflects light erratically, is practically useless for mundane decoration. It's also useless for many resurrection spells, which the diamon is theorized to "catch the soul's light" as well as acting as a tribute to the powers beyond.

A raw piece of gemstone is largely useless for most spells because of this.

Gemstones require expert cutting to become valuable, which take time and expertise.

Gemstones can be made into magical jewelries, which are a different application somewhere between making the gemstone into one-use weaponry and using them as spell focus.