Ask HN: How much data can be stored on a grain of rice?

While walking through Amsterdam (where I live), there was a lady who was able to write a name on a grain of rice. Surely, most of you have seen people like these in touristy places.

I was wondering: if you were to take #{something}, the size of a grain of rice, how much data could it reliably store (long-term, i.e. more than 1 month)?