Real deal super cool antique piece.  It is hallmarked on the chain near the clasp.  Those little suckers are so small that I cannot read them.  I did make out the French eagle stamp so I know this came from France probably sometime around 1900.  I tested both the chain and body of the pendant to confirm 18K gold.  It is a really good quality piece, even if it is dainty, that it might be a top notch maker over there.

The pendant section measures approx. (7/8") 21mm east to west by   (1 1/4") 32mm north to south The chain is 16" long. The Diamond dangle section measures approx. (1" ) 25mm north to south. 

The Natural Rose Cut diamonds measure in range from approx. 1mm to 1.5mm. The two Mine Cut Diamonds measure 2.2mm. The total approx. Diamond carat weight is 0.20 - 0.25ctw. The Diamonds have SI clarity with G/H color. 

It is not hallmarked, but has been tested and verified weighing in at 4.6 grams.   

Comes from a clean, smoke-free home.