“Diamond Fuji” is an atmospheric optical phenomenon where the sun appears to lie on top of Mount Fuji. This brief event only occurs at sunset and sunrise on a few days close to the Winter Solstice and can be viewed only if weather permits. The best sunset view is from the top of nearby mountain, Mount Takao. A lake near Mount Fuji sometimes creates a perfect reflection of the Diamond Fuji known as “Double Diamond Fuji.”
If you look at the first photo, there is no Diamond Fuji on the lake; whereas you can clearly see a double in the second photo.