According to China's national standards, the raw silk is classified into different grades: 6A, 5A, 4A, 3A, 2A, A, B, C, D, E, F and those that cannot be identified. Different grades mean different quality of silk.  

Grade A Silk

Including: 6A, 5A, 4A, 3A, 2A, A, with the grade 6A being the best.
Grade A silk, the best silk, has long threads, contains almost no impurities, and has a pearly white color with a healthy sheen. 
Grade A silk has very little silk lumps.  You'll feel warm all the time. A silk product without lumps can maintain its original silk batting structure, which increases the longevity of the silk.

Grade B Silk

Grade B silk contains more lumps than grade A silk. Grade B silk has some impurities and requires chemicals to make the silk look pearly white. Grade B silk has short threads. Compared to grade A silk, grade B silk feels rough and doesn't have a natural sheen.

Grade C Silk

Grade C silk has short threads, many impurities, a cotton-like texture, and a yellowish color. Actually, grade C silk is the innermost layer of silk. It contains the oil of the silkworm, which makes it look yellow and often has brown spots. 

Grade D, E, F

These are poor quality silk fibers that aren't suitable for silk bedding and silk sleepwear. They're normally used for silk carpets, silk curtains, silk fans, etc.