When the light from the sun aligns with the top of Mt. Fuji it results in this diamond-like shine.
This beautiful view is visible from Mt. Takao, also Sunshine 60 Observatory in Ikebukuro – but this brief spectaular occurs twice a year.
It’s visible for a few days after the “best time” so if you miss it, you can still try to catch it!
For more information, check out a Tokyo travel guide.