Following my comments on the currency supported by Fiat, it is not considered a "stable currency" because of the instability of all the legal currencies that are used to support it and the inability of organizations to demonstrate demonstrably their own currency. currency availability to support those cryptocurrencies.
Next we have the junk backed by raw materials, of which there are now hundreds that use a wide variety and a range of goods to support themselves. The most popular is gold, however there are many other crioclea that claim to be supported by a variety of different metals and precious stones. In almost all cases of my research, charlatans have selected a single commodity to support their currency. More recently, it is Venezuela's attempt to create the Petro currency supported by the reserves of a specific oil field.
One thing that should be obvious here is that commodities have an even more unstable history of legal currencies and a wider range of possible manipulations to their value. In the same way that DeBeers artificially inflates diamond prices by accumulating quantities of diamonds from the market, there are similar activities underway in many other businesses. Some goods are also just a geological discovery, far from a collapse of value or at least a significant devaluation.
In supporting a cryptocurrency with anything, the challenge with raw materials is that you actually have to accumulate the asset somewhere. For DGX, they have a "safe house" that contains the gold they use to support their currency. This could potentially make it easier to prove that the stock exists or not. The crypts supported by the currency of Fiat only need to produce bank account balances – currencies supported by assets must have the amount of hard assets in a secure warehouse somewhere.
It would be easier for a crypt supported by the fiat to diversify the banks and protect their support from possible robberies than large quantities of silver or platinum. In addition to having faith that stocks of goods exist, you must also have confidence that they are stored securely and will not be subject to being stolen in one big shot; the country will suddenly change control due to a coup or that there will be a method to liquidate the resource in case of need.
While there are some positive aspects of a crude supported by commodities, there are many more negative aspects and a much greater chance of volatility in value or the complete collapse of cryptocurrencies supported by raw materials. So, if you can not accept that a crypt supported or fiat is valid as stable, then a crypt resting on raw materials or based on raw materials is even more external to the definition of "stable currency".