Furniture, Wood and Cabinetry Finishing - Trying to make a cocktail table out of stone

I subscribe to Metropolitan Home magazine and in the July/Aug edition on page 85 I came across a table that I thought would look great in my living room. Do you know how they assembled this piece?I would like to make it 48 x 72 and think the base should be 12" tall and maybe 36 x 60. I don't know if the pebbles are in cased in a plexiglass box because it looks so perfect. I heard plelxiglass bows so I'm not sure. Then it appears there is a 1" gap or so before the glass is laid on top. Any ideas on how to assemble this piece? Thanks in advance for your suggestions.

Not familiar with the magazine, but you mention stone, then plexiglass, then glass. Not sure what material is where. But all need support in the middle. An 'X' frame or cross supports are needed. Nothing that size will support itself, for long.