Mana Stones

Mana stones is a term used for rocks that are able to hold and bind magical power to them. Only scholarly mages - and there are very few of them alive and practicing their craft - are able to identify their different kinds and properties. To everyone else, they look and feel like shiny rocks. Mana stones come in all kinds of shapes, sizes and colour and in their raw form they are reminiscent of opals - which is what scammers use most often to pretend their creations and jewels have any magical properties. 
Mana stones are incredibly rare, because you can only find them in places that have been impacted by extremely strong and raw magical power. Think magical disasters and explosions or other events of high impact. The high amount of magical energy required to create them is capable of warping reality and transforming landscapes. Places of such catastrophes are dangerous to mortals and mages alike and only a fool would try to go there in a search of these precious stones. However, mankind has never suffered with a lack of fools, so despite it being so dangerous, there are those who were able to find genuine mana stones, retrieve them and make a fortune. It is far from easy as possible.
These stones are most often used to make talismans and little protective runes that are hidden in the construction of new houses to protect its occupants and grant them a happy future. Bigger stones are usually locked away in the vaults of various nobles and each of those stones have some sort of a tale or a powerful mage attached to them. Sometimes literally as magic falls in and out of favour as years go by, and there is a tendency to lock up anyone who can weild it. Most often, the talented lock themselves in towers volunterily, because at least they will have a calm environment to work in and won't have to bother actually talking to other people. More than it's necessary.
There were many attempts to create mana stones artificially, with little to no success, but many explosions of various kinds. Should you be a rich noble, it is advisable to build your wizard's tower as far away from your own chambers as possible.