# Why is ninhydrin stable as a geminal diol? [duplicate]

The following image depicts the structure of ninhydrin - $$\ce{C9H6O4}$$

Ninhydrin, as you see has two hydroxyl group attached to same carbon. In general, geminal diol (or gem-diol, for short) are unstable and tend to lose a water molecule with formation of aldehyde or ketone (depending on the carbon position).

But, Ninhydrin isn't unstable. Now, I have the following questions:

1. If gem-diols are unstable, then why Ninhydrin isn't? Plus, this post depicts an image (marked as D) which shows the keto-form of Ninhydrin.

2. If there are any more compounds like Ninhydrin which shows this kind of exceptional behaviour, then how one can identify if they are stable or not?