You can use epoxy for seams (where there is no structural integrity required), but I prefer Polyester resin. It is easier to use for the novice, cleans up easily and it is just plain easier for you to make a good seam.
With either glue if you have never done this before and you need a study guide get this instructional dvd and watch the 25 minute segment on seams:
http://www.defusco.com/How-To-DVDs-&-Tool-Packages/c654/p2299/Fabricating-Stone-for-the-Do-It-Yourselfer---Glues-&-Sealers/product_info.htmlFor the poly resin I would use this:
http://www.defusco.com/Glues:-Epoxy,-Polyester,-etc.-Polyester-Resins/c82_501/p1432/TENAX-KNIFE-GRADE-POLYESTER---1-LITER/product_info.htmlwith this color kit:
http://www.defusco.com/Glues:-Epoxy,-Polyester,-etc.-Coloring-Kits/c82_504/index.htmlIn order for me to make a recommendation on your crack repair, does the crack go all the way through the stone or is it a fissure? Is it spreading? If it is a hairline crack you would repair it differently than if you have a clean break, so please tell us more. If you can post some pictures.
Glad to be of service,
Mark