Vickers Microhardness number (Hv) are various for different sides of crystals. For example, for crystals with cubic lattice hardness of a side (111) is higher, than hardness of a side (100). Hv strongly depend on orientation of a diagonal of a pyramid concerning crystal directions on face of a crystal.

Certainly, the crystals with microdefects hardness have less.

Thank you Prof. Sergey

i agree with Sergey Bogdanov. but you should also keep in mind that what indentation load you are applying, what indenter you are using, how does plastic deformation take place, what is the young's modulus of your material, what is the crystal size for certain plane, indentation depth, Thermal drift correction of your instrument and etc

Vickers Hardness test is not the way to determine the mono-crystallinity or poly-crystallinity of materials. Real Time 2D Bragg XRD Microscopy is the best way. You folks are in the "stone ages". What's up? Why don't you just drop the crystal and check the fracture mode? In fact in certain crystals just the Vickers indent alone is sufficient to create and propagate a crack.

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See if you can wrap you brains around this stuff. There are two such devices in existence today. One in NJ USA and the other in Bengaluru India:

http://www.youtube.com/watch?v=IU0m4yI7D-k&list=PL7032E2DAF1F3941F

You could use polaroid film and a simple laboratory X-ray source to determine the quality of your crystal in minutes non-destructively. As a crystal grower myself, "been there done that"!

BTW I'm neither a Dr. nor Prof.!

There are different techniques that you can use. XRD, EBSD, but microhardness vickers should be instrumented for obtain load-displacement curves and should be necessary the use of complementary techniques. You could try using XRD or EBSD.