RQD is an acronym for a technique that is used in geotechnical engineering principles in which that determines the quality of rock that was recovered when taking a core sample. It means rock quality designation and representative cross sections of the core sample must reach, or exceed 90- 100 mm in length for it to be considered excellent in quality. An RQD index is listed below . 

RQD is defined as the quotient:

RQD = \frac{(Sum of 10)}{ltot} *100%
(Sumof10) = Sum of length of core sticks longer than 10 cm measured along the center line of the core
ltot = Total length of core run
Classification Table

From the RQD index the rock mass can be classified as follows:

RQD Rock mass quality
<25% very poor
25-50% poor
50-75% fair
75-90% good
90-100% excellent

Jeffrey Havey