While it's not specifically stated in the rules, the basis for the different detection stopping power of these materials loosely corresponds with the ability of different materials to stop real-life electromagnetic radiation overall, including visible light and x-rays.
This is made most obvious by the special inclusion of lead, a material that is very soft, but is well known for being good at blocking x-rays due to its density, and not much else. (Density, not hardness, is the main factor for determining the ability of a material when it comes to blocking radiation.)
Brick, as well as tightly packed soil (but not surface soil or wood), has a density of around 2 g/cm^3 - close to that of most common stone. So one can probably classify brick as stone for these purposes.
In real life water and glass are almost as good as stone for blocking high-energy radiation, but they do let visible light through, so it makes sense for detection magic to penetrate them as well. But when it comes to determining whether a solid, opaque material is closer to stone or metal, it seems logical to extrapolate the rule based on its density.
(Most common metals hover around 7-9 g/cm^3, while lead is a high 11.34 g/cm^3. Incidentally, gold and platinum have densities even higher than lead and are better at stopping radiation, but are not typically used because they are expensive. The rules for Detect Evil/Good state "common metal", and one can argue that these are not common metals, but precious metals, while the rules for Detect Thoughts do not specify "common". Whether one wants to give a gold or platinum sheet the same magic-blocking abilities as a lead one is up to the DM's digression.)