If you have been exploring the world of Story of Seasons: Friends of Mineral Town, you will have noticed a cave on an island in the middle of the lake. This is actually Lake Mine, and getting access to it promises you all kinds of gems and treasures. You will need to wait to get into the Lake Mine while you can access the Spring Mine.
The Lake Mine can only be accessed during the winter. You will need to wait until the very first day of the winter season to get into the Lake Mine. The harsh weather will cause the lake to freeze over, and you will be able to cross it and go inside.
This mine works the same way as the Spring Mine. You need to smash rocks with your hammer and dig into the ground with your hoe to find the ladder to the next level. This mine will give you gems instead of ores and should be where you spend most of your time during the winter.
Because crops won’t grow, you will have a lot of free time to dig through the mine in search of gems. You can sell these for good money, save them, or use them at the blacksmith to make certain items. It is a good idea to keep a few of each type during your first winter and then sell all the rest to make sure you still have a good income source during the winter months.