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There are handful of divination spells that are blocked by barriers of a particular thickness and material, such as detect evil and good:

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

...and detect thoughts:

The spell can penetrate barriers, but 2 feet of rock, 2 inches of any metal other than lead, or a thin sheet of lead blocks you.

Would a wall of brick more than 2 feet thick block these spells (i.e. qualifying as stone or rock), or would that wall be a barrier that the spells can penetrate?

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Technically, fired brick (the thing you think of as a brick, as opposed to mudbrick) is a form of ceramic, cooked in kilns; but I don't think there's much benefit in thinking about this too hard or getting overly technical about what a brick is on a microscopic physical level.

If you have to choose off the list of "dirt, wood, stone, or metal", stone seems the only appropriate option to use. If you're looking at a mudbrick wall, you might treat that as dirt, but that's kind of up to the DM to decide.

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    \$\begingroup\$ +1 for "don't think too hard". The DM should be able to make a reasonable call as to how bricks should react, and if their call is unreasonable then no amount of complaining on stack exchange will fix that. \$\endgroup\$ – pllpnakjlx Dec 17 '19 at 9:08
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    \$\begingroup\$ Insurers normally group stone, brick and concrete as "standard construction". A geologist would probably regard brick as an artificial metamorphic rock. \$\endgroup\$ – nigel222 Dec 17 '19 at 13:05
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    \$\begingroup\$ I know archaeologists refer to fired brick as "CBM", or "ceramic building material", to avoid using the ambiguous term "brick", which can refer to fired brick, mud brick, or small cut stones. \$\endgroup\$ – Darth Pseudonym Dec 17 '19 at 14:53
  • \$\begingroup\$ +1 but really the list of choices provided is "dirt, stone, metal, lead". Wood is basically just dirt for the purposes of the spell, and lead needs its own category. \$\endgroup\$ – GrandOpener Dec 18 '19 at 2:56
  • \$\begingroup\$ Since brick is obviously not similar to metal at all, we don't need to decide what kind of metal the brick is most like, and anyway only lead is lead. Nothing is in the lead category except lead itself. Similarly, I mentioned both dirt and wood because they're very different materials despite having the same degree of blocking effect. If you're looking at a material and deciding which one it counts as, you don't skip wood! ("Bamboo is nothing like dirt, so it can't be that, but it's obviously not stone...") \$\endgroup\$ – Darth Pseudonym Dec 19 '19 at 3:13
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Yes and no

Bricks are made from clay (which is a type of dirt) mixed with a fiber (usually straw), then baked to dry it out. Detect Evil and Good would be blocked by 3 feet of brick, whereas, Detect thoughts would penetrate through brick. Spells do what they say they do and don't do what it doesn't say.

However, a DM could always forgo the actuality of what brick is and count it as stone.

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    \$\begingroup\$ Personally, I would count brick as both rock and stone not because of what it's made of but the fact that it's hard as a rock and would likely absorb a similar amount of magical energy (or physical energy of whatever given type) as a stone of the same shape and size \$\endgroup\$ – Himitsu_no_Yami Dec 16 '19 at 21:35
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    \$\begingroup\$ @Himitsu_no_Yami That's why I added the last sentence. \$\endgroup\$ – Eternallord66 Dec 16 '19 at 21:36
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    \$\begingroup\$ Modern bricks typically do not use fibre but they are fired at a much higher temperature so they are effectively turned to "stone" \$\endgroup\$ – Dale M Dec 17 '19 at 1:39
  • \$\begingroup\$ @DaleM Yes, but the typical brick from the time we usually have our settings, it was fibre. \$\endgroup\$ – Eternallord66 Dec 17 '19 at 14:13
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    \$\begingroup\$ @Eternallord66 Gonna disagree there. In the real world, fired (i.e. ceramic) brick was in widespread use by the Romans. They even had portable kilns so the army could make brick on-site for construction projects. The standard medieval-esque D&D setting equates to an era almost a millennium after that point. While mud brick was certainly still in use in areas that were dry enough that it wouldn't melt in the rain, most D&D brick is almost certainly fired clay. \$\endgroup\$ – Darth Pseudonym Dec 17 '19 at 15:00
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Most likely

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.)

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Geologically, yes.

Clay is just a suspension of sand (tiny rocks and minerals), water, and humus (organic material) within extremely fine-grained igneous rocks. Technically, clay is a stone material even before you do anything with it.

Over time, heat and pressure squeezes out the water, and the humus breaks down, its minerals (such as carbon) bonding with other materials (not unlike what happens in steel-making). This is how clay becomes sedimentary stone. Shale is a good example of this.

Firing a brick within a kiln just forces that process.

Practically, yes.

If it looks like a stone, feels like a stone, and clocks a Burning Fist guard in the head like a stone, then it might as well be a stone. Don't waste time at the table pondering this sort of thing.

Rock vs Stone

Same thing, seriously, I know my players were RPing dwarves but they're the same thing. Usually rock refers to the object and stone to the material, but they're interchangeable, can we please enter the dungeon, please?

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