How can you tell a fake black diamond?

Diamonds conduct electricity, and your local jeweler probably has a diamond tester that uses this characteristic to tell real stones from the fakes. You can take your black diamond in and ask that the stone be tested. Edit From Siddharth Nayak: Also, if a diamond tester tries to get payed for his/her services, there is a quicker, faster, and free way: simply hold the diamond out in the sun, and if it gets hot, then it is glass, if it stays normal temperature, it is a diamond...but then again, it might be simply coated with diamond and internally glass... -Siddharth Nayak