1  Red Synthetic Diamond Crystal and a  Pyrope Garnet
The two stones are held on the end of fibre optic light guides with Blutack or similar.
The red synthetic diamond  crystal was obtained at Tucson 1998.  The appearance was reminiscent of pyrope garnet.     The crystal  was mainly a cubo-octahedron with  further series of stepped faces  which related to other cubic crystal forms.
 

2  Magnetic Response
Filling the motel room wash hand basin, the red crystal  was floated on a small piece of torn-off bubble pack. Using a small rare earth magnet (neodymium boron ferrite),  the resultant raft could just be towed very slowly about the surface.   It is actually safer to use a small wooden raft, but in either case,  check for a response of attraction before adding the specimen.
The magnetic attraction  indicated  a small level of  response  emanating from the inclusion content of the  synthetic diamond.
 

3  Bright Metallic Inclusions in Synthetic Diamond
Large  (pique 1 or worse), inclusions of nickel/iron in synthetic diamond will produce a dramatic response to a rare earth magnet.  The  inclusions are typically bright metallic in appearance.  They lack the  halo fractures or rosettes (black or silvery)  which typify the inclusions in  many a natural diamond.  The  colourless faceted synthetic diamond (0.18ct)  shown, will jump approximately  one centimeter to attach to a rare earth magnet, but  the large nickel iron inclusions seen entrapped by the growing crystal explain this dramatic response. The stone grade is clarity grade Pique 2.

4  Positive Magnetic Response
Here a synthetic yellow diamond shows a positive attraction to a ferrous neodymium boron Hanneman Magnetic Wand.   The nickel /iron inclusions in this synthetic diamond (courtesy DeBeers)  are approximately diamond clarity grade SI
The responses  in Figs  47, 48 and 49  are positive for synthetic diamond.
 

The big question is.  Do natural diamonds exhibit any magnetic response of attraction?
Diamonds themselves produce no such magnetic response, natural or synthetic.  With help from Dr J Harris of Glasgow University, we looked at natural diamonds with large inclusions of iron sulphide eg Pyrrhotite and Pentlandite (chemically related to pyrites). Using the raft technique in a large container of water,  it was just possible to demonstrate the slightest response to a rare earth magnet.  This observation does not  contradict the above underlined statement as follows:-

5  Pique 3  Inclusions in Natural Diamond.

Typical black halo and rosette inclusions in natural diamond  are often iron sulphide minerals and mineral films.  Even large inclusions such as the one shown produce only the slightest magnetic response, and would require  suspension in water or air to observe the merest attraction response. In a synthetic diamond, such a sizeable inclusion  would show a  very dramatic attraction response to a rare earth magnet as in  Additionally, the synthetic's inclusion would normally be silvery metallic in appearance.

Small inclusions in synthetic diamond (finer than pique 1) may be seen as dust clouds, or tiny pellets. These will still show a positive  magnetic attraction on water if not in air, whereas such minimal inclusion content in natural diamond would produce neither  of the  magnetic responses described above.  Most synthetic diamonds available to the trade at this time are from Russia, and available in various shades of yellow and colourless. Mostly a magnetic response of attraction can be observed.  The Japanese Sumitomo synthetic diamonds are invariably yellow shades. Those seen by the writer exhibit no magnetic response in spite of having small ( clarity grade SI ) metallic looking inclusions. These Japanese synthetic diamonds  may be identified by their fluorescence, grain lines aligned to  cube faces, and strain birefringence patterns.

An excellent wall chart from GIA sets out a comprehensive overview of the distinction between natural and synthetic diamond.   "A chart for  the separation of natural and synthetic diamonds" Shigley J et al , Gems and Gemology Vol 31 No 4 . Winter 1995

Red Synthetic Diamond Absorption Spectrum
Using a small OPL diffraction spectroscope, and a pen light torch to study the red diamond in Figs.1 and 2, it was immediately possible to see a powerful band at approximately 637nm,a weaker 594nm and three weaker bands in the same area.
Here was confirmation of irradiation and annealing , and compares with the absorption pattern seen in treated pink natural diamonds.

6  "Natural" Pink Diamonds
The central pink diamond is a macle crystal of natural colour from the Argyle Mine in Western Australia.  The specimen exhibits no absorption lines to a  spectroscope at room temperature.   The two faceted round pink stones are colour enhanced by irradiation and heat treatment, and exhibit very similar spectra to the synthetic red treated diamond as follows:-
 
 

7  Three Diamond Spectra - Diffraction spectrum left, prism spectrum right.
The top two rows show  the same suite of lines  in the yellow/orange zone observed in the two faceted stones in Fig 6.
The bands about 595nm and 637nm lines provide visible evidence of annealing after initial irradiation.  The line at approximately 575nm is interesting, as it is reversible from absorption (top row) to emission mode (second row)  - in the manner of a ruby 693/694 doublet.
The bottom row spectra stem from the syntheic red treated diamond, and produce a similar suite of lines  to the natural stones which have been colour enhanced.
 

The absorption pattern in the red synthetic stone was so evident, that it could be seen with a penlight torch and a 2" (50mm)  OPL  diffraction spectroscope in the Tucson Motel room.  I think it permissable  to mention here that Colin Winter's OPL spectroscope has been such a  remarkable and inexpensive boon to gemmologists worldwide since 1976, that it is now blatantly copied.  The measurements were achieved later with a Jena spectroscope from Eickhorst, and a measurement control system supplied by Nelson Gemmological Instruments.