If you are questioning yourself then slow down .... yes any time you place a foreign object into your body there is potential for harm. Clean it, lube, and don't force it or injury can result - ten inches for a rookie is taking on a lot. I personally enjoy my own fingers - two fold sensation.
No it's not harmful, at all, and can be very intense when done correct. Especially if you can work your prostate gland! Just inside, about an inch or 2 and towards your stomach (it will differ slightly by person) You will know once you have found it.
