From the Official Handbook of the Marvel Universe:
Emma can now transform her skin and hair into a diamond-hard form at will. In this form, Emma is nigh indestructible except for one small flaw that, if exploited, can cause her to shatter. Whilst in her diamond form, Emma's telepathic abilities are suppressed.
Emma Frost can transform her entire skin and hair into a flexible diamond. She is nearly indestructible in this form, except for a few things that can cause her to shatter, such as a diamond bullet.
Emma Frost is mostly indestructible whilst in diamond form, but there have been some instances that show her diamond form has weaknesses.
- Esme shot Emma with a diamond bullet in a vulnerable spot that caused her form to completely shatter. (New X-Men #139)
- A Purifier stabbed a dagger into Emma's arm whilst in diamond. Though painful, it did not cause her to shatter. She transformed back into human flesh and used an arm sling to heal. (New X-Men: Childhood's End)
- Mr. Sinister sliced off her arm while in diamond form. She displayed no signs of pain until she reverted to human flesh. A molecular weave machine was used to reform her arm. (Uncanny X-Men v2 #3, Uncanny X-Men v2 #8)
Whilst in diamond form, Emma cannot access her telepathy nor can others use their telepathy on her. Although there have been inconsistencies, she is completely immune to telepathy.
Emma Frost claims she has no empathy, save for a "cruel sense of humor and a cold, hard heart" when in diamond form.
She is sometimes seen transforming into diamond form when dealing with emotionally tough situations, such as facing the death of a student.
Empathy is defined as the "the ability to understand and share the feelings of another." So, Emma has displayed empathy for herself while in diamond form. For example, Emma displayed fear when Danger once threatened to drill into her forehead. (Astonishing X-Men: Dangerous).
There have been inconsistencies with her lack of empathy whilst in diamond form, though. A statement that she has varying degrees of numbed empathy is probably more accurate than saying she has no empathy at all.
Whether Emma Frost has superhuman strength or not whilst in diamond form is an inconsistent feat. (More to come).