I use JB Waterweld and (pick a brand) SuperGlue Gel.
Mix up enough JB WW for the job, then divide in half. Mash the first half into the rock where you want to place the frag. Press the frag into the other half, then remove it. Put a drop or two of SG in the depression left by the frag and put frag back in place. Put another couple drops of SS on the base of the frag/JB WW combo and then mash it all into the other half of the JB WW already in place on the rock. You may have to hold it for a few seconds to allow it to set, then let it fly, be free.