Gemdle

Gemdle is built for people who want to learn to recognise stones by what they can see. The daily round has five questions: a photo of a real specimen, many from Wikimedia Commons with their photographers credited, and four names to choose from, such as Ametrine, Stichtite, Watermelon Tourmaline and Iolite.

After each answer the game explains why it was right or wrong, pointing to observable traits: colour, lustre, hardness on the Mohs scale and crystal habit. A flame counter keeps your streak of days played, and you can share your score.

Beside the daily five there is an unlimited quiz, an identifier that ranks likely matches for a stone you own from the traits you can observe, and an A to Z guide of more than 250 stones with photos and properties. It sticks to physical properties rather than crystal-healing claims, and suits rockhounds, collectors and anyone curious about minerals.