"It tells you what the metal is. It does not tell you what to do with it."
The instrument, literally
A touchstone is a piece of fine-grained dark stone — historically Lydian schist or a similar slate — and the technique is about as old as coined money. You rub a sample of gold across it and it leaves a streak. Then you rub a set of reference needles of known purity beside it, and you compare the colors. The needle whose streak matches tells you roughly what you are holding. Add a drop of acid and the comparison sharpens: lower-purity streaks dissolve, higher-purity ones survive.
Notice everything that is not in that process. The stone does not know the spot price of gold. It does not know whether you inherited the piece or are about to sell it. It cannot become urgent, cannot flash, and cannot decide that today is the day. It answers exactly one question — what is this metal? — and it answers it by comparison against a known reference set, which is a detail worth holding onto, because it is the whole architecture of this app in one sentence.
Why that becomes a product brief
A market valuation engine faces a permanent temptation. It knows a lot of numbers, some of them dramatic, and the entire surrounding industry is built on turning those numbers into action. A cyclically adjusted price-to-earnings ratio sitting near the top of its own recorded history is interesting. The distance between that fact and "so you should sell" is one sentence of prose, and almost every product in this space writes that sentence.
Touchstone does not, and the refusal is not squeamishness. It is a claim about where judgment belongs. The tool's job is to make sure a human decides from accurate, calm, well-sourced information. The moment it starts producing the decision itself, it has quietly taken on a role nobody granted it and cannot be held to.
The reference set is the whole trick
Here is the part that carries the rest of this quest. The streak on the stone is meaningless on its own. It is a color. It becomes information only when it is compared against needles whose purity is already known — and it is exactly as trustworthy as that reference set is.
Every gauge in this product works the same way. A price-to-earnings ratio of 43 means nothing by itself. It becomes meaning when you can say where 43 sits inside the same series' own recorded history: near the top, near the bottom, unremarkable. That is a percentile, and a percentile is the reference needles. Which raises the question the entire middle of this quest is about: how long is that history, where did it come from, is it complete, and does the surface say so?