Someone gave you a stone and didn't say what it was. Or you bought a bag of tumbles and half of them look alike. Start with the colour — it narrows 37 stones down to a handful in one click, and each result links to a full guide with its markings, chakra and care.
Colour only gets you so far
Plenty of stones share a colour, and several are routinely sold as each other — dyed howlite is passed off as turquoise, glass as amethyst, and man-made opalite as opal. If you want to be certain, our article on five tests you can do at home covers the checks that don't need a lab: weight, temperature, bubbles, wear and the streak test.
Still not sure?
Work the other way round — start from what you want rather than what you have. The two-minute crystal quiz asks five questions and names a stone; the full library lists all 37 with meanings and care.