Flagstone texture simulates a mosaic of stone tiles.  The stones appear to be selected to fit together but have random "grain" direction.
                 
Parameters :

Position : Position of the texture.

Scale : The texture is scaled by this value.  Use these parameters to scale or stretch the texture in one or more directions.                

Thickness : Nominal thickness of the stones.

Mortar Width : The nominal width of the mortar lines between the stones.

Jitter : Noise applied to the stones.  Jitter value of 0 will not distort the stones.  Higher values for Jitter distort the stones more and more.

Twist : This parameter represents how "tightly wound" the Jitter is.  Higher values here make the Jitter "grittier".

Meander : This is a second noise that is applied to the stones.  This is designed to push and pull the stone mosaic on a larger scale.  Used with Meander Size to produce effect.
                 
Meander Size : This is the scale of the Meander.  Larger values here cause the effect to be applied to the stones on a larger scale or area.

Roughness : The stones will have a characteristic roughness.  A value of zero here makes the stones appear smooth.

Rough Size : The scale of the roughness.  Larger values here make the stones have a coarse texture, smaller values give a finer texture.

Rough Grit : This parameter affects the detail present in the roughness. Higher values here make the texture "grittier".

Rough Stretch : The Rough Stretch parameter allows the roughness of the stones to have a grain direction.  Each stone will have its own grain direction.  The higher the value here the more directional the texture becomes.

Notes : The Flagstone gives the appearance of randomly shaped stone tiles pieced together to form a mosaic floor or wall.  Noise is introduced with the Jitter and Meander parameters.  The pattern can be distorted slightly to improve realism.  Progressive distortion can produce interesting decorative patterns.