I just pulled mine apart. There is a metal disc in the base of the gyro. The base on it's own weighs 6g. Very impressed with the construction of the gyro. The board can't rattle in the case because it is clamped between the top and bottom case. The soldering also looks excellent. 32 bit processor and a single InvenSence MEMS sensor. I've taken some pictures:
More closeups attached.