Teeth have a hard surface layer called enamel.

牙齿有一层叫做釉质的坚硬表层。

《牛津词典》

Most Roman pottery is light and smooth to touch and very tough, although, like all pottery, it shatters if dropped on a hard surface.

大多数罗马陶器轻巧而光滑，摸起来很硬，但像所有的陶器一样，如果掉在坚硬的物质上就会碎裂。

youdao

Try switching running surfaces (if you've been running on a hard surface, try a softer one or vice versa).

换一种路面去跑步(如果你跑步的路面很硬，就应该试着在软一些的地方跑，反之亦然)。

youdao

If you want to dig beneath the hard-scrabble surface of this desert oasis, start with the OriGen Experience.

BBC: Discover life in the Nevada desert

Unfortunately, tragedy can strike with as simple an act as brushing one's elbow across the table, and that's all it took for me to knock my smartphone right onto a hard surface -- facedown in a perfectly horizontal position (the worst imaginable angle).

ENGADGET

He has a well-honed sense of presentation and play--a popular dessert is a "planet" with a raspberry core, grapefruit-cream surface, and hard-sugar atmosphere (his blueberry satellites didn't pan out).

FORBES: Debut