Tooth enamel is ~5 on mohs scale. Quartz, the most common kinda rock (afaik), is ~7. You're correct that enamel is harder than steel though, since it's ~4? Disclaimer: all of these numbers are from a quick google search.

Fun fact, this is why we know ancient people used to make bread - the way they separated wheat from the chuff included dropping it on the ground, which led to sand being in the bread and consequently destroying their teeth over a lifetime of eating it.