Most likely some sort of diamond. Some diamonds of the rarer kind go for insane amounts a gram. Of course, the problem is they are often faked, so the market is most likely inaccurate, rigged, since they are very hard to discern from the real deal. See:
https://en.wikipedia.org/wiki/Synthetic_diamondAnother reason to hold Bitcoin above other assets.
There's no good way to tell if a commodity is "expensive" or not, unless you're looking at previous prices. Oil isn't at its all-time high, nor is bitcoin, nor are precious metals. If you were talking about the stock market (which you're not), you could look at price/earnings ratios, and that would tell you if a stock was expensive because there are yardsticks people measure that by. Not so in the commodity market as far as I know. If anyone can educate me about that, I'd love it.
Bitcoin is pretty expensive IMO if you compare it to 2010. This is relative, though. If it hit $100k in 2025 we could look back and say it was cheap in 2018. It's kind of a weird question.
The most expensive stock has to be Berkshire Hathaway A. I remember in 2008 it had a massive crash from $150,000 to $75,000 and people were tripping and thinking everything was going to 0:
Ten years later and it has finally crossed the $300,000 line:
You can look at that all time graph and get an idea of how the BTC all time graph will be in 20 years, the good news is we can get at near the bottom.