It's not that the gravel is unsafe, it's just over longer periods of time it will loose it's color generally and possibly chip. However I use the Aqua Culture brand from Wal-Mart (because I'm cheap lol) and I've only seen a few chips, just take them out and you're all set ^^
However I am in the process of switching to black sand because I find it's a lot easier to clean, especially with planted tanks since you don't have to vacuum it out like you would gravel. You just go in circular motions over the top, bout half an inch to suck up the poo and debris. Yeah, you'll get some sand up the siphon but you'll learn how not to do that with time ^^
It's also easier for your plants to root in, but I've done planted tanks with gravel just fine
The only thing I really suggest not to do is half gravel half sand. Eventually your gravel will get into your sand and visa versa and it's just such a pain to keep up with. And then you'd really be sucking out a lot of sand when you clean the gravel part.