This study analysed secondary school teachers' self-reported learning during their participation in a long-term, non-formal scientific activity in the form of a crystal-growing contest. Therefore, the purpose of this research is to explore in-service teachers' experiences and describe the different types of learning that they report they have acquired due to their participation. Data were collected over a period of four years using an open-ended questionnaire. Qualitative methods were used for the data analysis to construct categories and identify emerging themes. The outcomes of the analysis were the identification of several types of learning gains that highlighted some of the potential benefits of introducing non-formal environments as an option for in-service teachers professional learning.