Chiapas amber – fossilized pine resin that's around 30 million years old – is known for its clarity and diverse colors. Most is mined around Simojovel, north of San Cristóbal. This small museum explains all things amber (with information sheets in English, French, German, Japanese and Italian).
Note that a number of nearby jewelry shops have appropriated the museum's name, but this is the only place around that's housed in an ex-convent – you can't miss it.