How Rocksday works

Rocksday is a daily rock and mineral guessing game with a reference library. This page explains exactly how the puzzle works and where every fact and photo comes from.

The daily puzzle

There is one puzzle per UTC day, the same for everyone, drawn from a fixed schedule of about 300 specimens that is only ever extended, never reshuffled. You get 6 guesses. The photo starts zoomed in three times and blurred, and every guess zooms out and sharpens it. Each wrong guess unlocks the next clue, in this order: type, color and luster, Mohs hardness, crystal system (minerals) or texture (rocks), then notable localities.

How guesses are compared

Type is green for an exact match and yellow when both are minerals or both are rocks. Hardness is green when the midpoints of the two Mohs ranges are within 0.5; otherwise an arrow shows whether the answer is harder or softer. Color is green when the color sets match, yellow when they share a color. Crystal system or texture is green only on an exact match.

Where the specimen data comes from

Properties — Mohs hardness, luster, streak, crystal system, chemical formula and specific gravity — are compiled from standard mineralogy references and cross-checked against public data such as Wikidata and the IMA list of minerals. Rocks are mixtures of minerals, so their hardness is a practical approximation, written as "about". Identification tips and lookalikes are written for field use, and facts that can't be verified are left out.

Photos

Every photo is a freely licensed image from Wikimedia Commons (CC BY, CC BY-SA, CC0 or public domain), credited to its photographer. Each specimen has one library photo and up to three puzzle-only photos, all reviewed by hand to make sure they show a real hand sample of the right specimen.

Quick tests on comparison pages

Comparison pages derive their quick tests from the data: scratching glass (Mohs 5.5), a fingernail (2.5), a hardness gap of 1.5 or more, fizzing in dilute acid (carbonates), magnetism, streak color, heft when specific gravities differ by 30% or more, metallic luster, and whether one is a rock and the other a single mineral. Only tests that actually separate the two are shown.

The identifier

The identifier scores every specimen against six optional home tests — metallic look, scratch hardness, main color, fizzing in acid, magnetism, and crystal versus rock — one point per matching answer (half a point for weak fizzing), and ranks the best matches. It's a starting point, not a lab identification.

Corrections

Spotted a mistake? Email hello@rocksday.com and it will be fixed.

Last updated 2026-10-23 · About Rocksday