100% silicone

Is there such a thing as silicone epoxy, or do you mean just regular silicone caulk?

I prefer some sort of mechanical fastener instead of floating the shelf in place with epoxy or silicone. Personal preference.

I've cut grooves in the side and back of the shelf edges and in the side and back walls of the niche tile, then used a coin in the groove to lock the two together.

Same idea as using a biscuit joiner when wood working.

Pennies (coin) are the biscuit. Epoxy is the glue. Mask off the tile with painters tape or equivalent to prevent epoxy drips from mucking up the tile.

Set the shelf with a slight pitch for drainage.

Pennies as tile biscuits. What will you think of next??? :-)

Mongos method is very valid.......and how i would approach a bigger heavier install... minus the copper penny that when gets wet may discolor?? in a shower/stone environment??

the epoxy itself into grooves is mechanical enough. for your small application that will never see any real bearing load....

.hopefully if you go that route youre good with an angle grinder and or repairing your porcelain.....( insert silicone method here)

If you use epoxy, you want a 2 part marine grade Fast set ....

Tundra, can you describe the silicone method? Is the product the same as 100% silicone caulk, or something else? It seems like silicone caulk might be too flexible to hold a heavy quartz shelf?

For epoxy, would we need to cut grooves, or can we simply epoxy the quartz shelf to the raw tile surface since it won't really be load bearing? The biscuit-type method sounds great, but might be over the heads of our quartz and tile guys, who seem to like simple solutions... any advice on what type of contractor we could call who might have the experience and tools for the biscuit job (or for cutting a groove if that is necessary?

Thanks everyone for your input!

To be clear , I am not discountimg Mongoct methods / stone trade skills

I would relax about the weight.! and focus on scribing the shelf just right and go ahead and groove the side like Mongoct said and shave a 1/16 off all sides of scribe and silicone it in place .

use a quality GE brand 100% silicone .

sleep easy!

Penny is encased in epoxy. It sees no water or air. If worried you can go with a stainless steel washer.

To cut the groove for the biscuit (coin), use a diamond blade on an angle grinder. Any tile guy should have those.

still conflicted?

Leave a slot? Are you kidding me? The niche should never have been put together without both shelves being there and ready to install. best way to do it is to set them on top of the edge of the side pieces of tile in the niche. Mechanical fasteners mean you're going to poke holes in the waterproofing in the niche, and surface applied adhesives are taking a real chance. If it were me, I'd demand the niche be disassembled and reinstalled with the shelves properly. No negotiations.

Silicone is really strong. I think Tundra's idea will work but next time set it in place and support it with the tile.

I also like the GE II brand silicone the most. Latsil Second from Laticrete.

Why not replace the stone middle shelf with a lighter-weight tempered glass shelf, as a compromise, with a silicone adhesive?