Most glasses block some amount of UV light, particularly the "harder" UV like UVC and UVB. UVC is not really relevant in the context of sunlight at normal altitudes, as the atmosphere, in particular ozone in the upper layers, filters it out.
Now, for the UVB and UVA parts that e.g. suncream needs to block to protect your skin; what about the glass that protects your eyes?
Let's first define the UV-regions:
- UV-A goes from about 315-390 nm, has longer wavelengths and less energy and does not penetrate as deeply into tissue
- UV-B goes from about 280-315 nm, has shorter wavelengths and thus more energy and does penetrate deeper into tissue, can cause damage there (sunburn, skin cancer)
- UV-C 100-280 nm, really high energy UV. Very damaging to tissue but usually not an issue with sunlight.
If we look at the following chart from Edmund Optics (a reputable seller of optical equipment and related fields)
we see transmission curves for different types of glass. As you can see, all of them start to let less and less light through the deeper we go into the UV. In particular, B270 seems to be a common mineral glass used in corrective glasses. It lets about 90% of 360 nm (which is UV-A light) through, and about 50% of 320 nm light, which is almost in the UV-B region.
So it lets some UV light through, but not all of it. That means that a ordinary, untinted pair of glasses is still better sunlight protection than no glasses, but by far not as good as sunglasses that follow the ANSI Z87.1 or UV400 guideline (explained here), which lets virtually no light below 380 or 400 nm pass through.
Addendum: for your application - you said you saw "sunglasses fitted with "mineral glass lenses"" - I think the white, untinted glass alone would not necessarily offer enough UV protection, depending on the glass type. As we can see, there are glasses that offer more UV blocking, but these glasses are not really that common to the best of my knowledge. You didn't tell us if the glasses were also tinted, which, again depending on the technique used, might block UV. Your best bet is to look for any kind of certification according to respectable governing bodies like the FDA in the United States or the CE sign in the European Union.