These were made using output of a custom C program, which was then converted by Mathematica for rendering with POVRay.
Ruby glass trefoil 640 X 512 JPEG (27 Kb) 1280 X 1024 JPEG (149 Kb)	Ruby-studded jade trefoil 450 X 450 JPEG (40 Kb) 900 X 900 JPEG (134 Kb)	Gaudy gem-studded trefoil 640 X 512 JPEG (103 Kb) 1280 X 1024 JPEG (369 Kb)
"Knot dancing" --- A View from Trefoilamadore 1600 X 1200 JPEG (362 Kb) 800 X 600 JPEG (102 Kb)	"Knot in front of the kids" --- a slight revision 1600 X 1200 JPEG (1122 Kb) 800 X 600 JPEG (307 Kb)	"Knot on the Escher parquet!" 1280 X 1024 JPEG (321 Kb) 640 x 512 JPEG (81 Kb)

These were made with a custom C program and the aid of ImageMagick. The texture patterns were first generated in Mathematica, then filled by hand using some woodgrain images stolen from the web.
Woodgrain trefoil using filled Escher patterns 600 X 600 JPEG (109 Kb) 1200 X 1200 JPEG (259 Kb)
Woodgrain trefoil using filled "Newfaux" (don't ask) pattern 500 X 500 JPEG (67 Kb) 1000 X 1000 JPEG (144 Kb)	500 X 500 JPEG (75 Kb) 1000 X 1000 JPEG (173 Kb)	500 X 500 JPEG (76 Kb) 1000 X 1000 JPEG (194 Kb)

Made using the same tricks as the glass trefoils above, the figure here is a POV'ed-up version of a favorite knot from years ago. I believe the knot people call this one "7_2" (the trefoil being a "3_1").
1280 X 1024 JPEG (740 Kb) 640 X 512 JPEG (206 Kb)	1600 X 1200 JPEG (563 Kb) 800 X 600 JPEG (146 Kb)