For all artifacts:
(Recursive Depth: 3; Image Size: 442; Global Ambient: 1.0; ; Adaptive Anti-Aliasing Depth: 2, soft shadow)

For all glossy reflection artifacts:
Glossy Reflection Glossiness: 5; Samples: 5 

BRDF function parameters:

Ward:  Roughness in X: 0.05;    Roughness in Y: 0.3; Specular Reflectivity: 0.05; Diffuse Reflectivitiy: 0.4

Lafortune: Cx Parameters: -1.0; Cy Parameters: -1.0; Cz Parameter: 0.95; Cosine exponent: 20

Cook-Torrance: Surface Roughness: 0.08;  Index of Refraction (real part): 1.6; (imaginary part): 0.2; Specular Reflectivity: 0.6; Diffuse Reflectivity: 0.4;   

กก

กก

Above left artifact is ray tracing without glossy reflection. 

Glossy Reflection without BRDF

Glossy Reflection with
Ward BRDF

Glossy Reflection with
Lafortune
BRDF

Glossy Reflection with
 Cook-Torrance BRDF