Diamond is perfect, as it does not lose its shape. It does not chip or scuff like saphire. Since it maintains smooth shape, it is ideal for playing nitro-celluloid discs. They should never be played with a steel needle. A saphire will do fine. Saphires, though used a lot on 78 rpm records, are not the greatest for 78 rpm shellac records, as some of the 78 rpm records will have inevitably been played with steel needles, which leave bits of steel in the grooves. These bits of steel chip the saphire. This condition is the only one that renders saphire less than desirable. Saphire is just fine, though, for home recorded discs and vinyl discs.

That said, your diamond will be fine. As for the size, unless the recording artist wrote what size needle they used on the label, you cannot be sure. Typically 33 rpm records are recorded with 1 or 2 mil needles. This being an amateur recording, you can not be certain. Typically 78 rpm home and professional recordings are made with 3 mil needles. You can try your 33 rpm needle (microgroove 1 or 2 mil) on the disc and see how it sounds. If you are able to obtain a 3 mil needle for your phonograph (or if it has tandem needles), you can compare the sound quality of the 3 mil and 2 (or 1) mil sizes. If the record sounds fine with a microgroove needle (1 or 2 mil), then it is likely cut with a microgroove needle. If the sound is somewhat floating, and the needle sounds like it is actually floating around in the grooves, and the sound improves with a 3 mil needle, then it was definitely cut with a 3 mil needle. Examining the grooves and comparing them with those on a professionally recorded 33 and 78 will likely disclose which size was used. As the record may have been cut a bit deep or shallow, sight may not disclose the size, and the groove may appear to be what it is not. All in all, whatever sounds the best and doesn't seem to do harm to the record will do.

The reason why I say 1 or 2 mil for the microgroove needle is that originally, when microgroove records came out (Columbia LPs and RCA Victor 45s), they used a 2 mil needle. When stereophonic records came out, they started using a 1 mil needle instead. 2 mil records can be played with a 1 mil needle, but one of the reasons why the record jackets of the stereo records said that the record cannot be played on monophonic equipment is that the 2 mil needle on the monophonic equipment will widen the groove of a 1 mil record. Actually, for the average listener, no damage is done. Only if your records are played on fine reproducing equipment is the damage noticeable.

Back to playing home recordings:

Lateral (side to side) tracking force should be gentle. These records were often played on the old twisting thumb screw cartridges, so your cartridge is likely better than the above condition. If the needle is rather rigid when pushed side to side, then your cartridge may be a bit stiff laterally. If the following conditions may be met with your phonograph, you likely have proper lateral tracking force, as the following conditions cannot be properly employed unless the above holds true. Vertical tracking force should be no more than 14 grammes at the most. 7 to 2 is preferred, whatever is recommended by the manufacturer of your phonograph. NEVER use too light of a tracking force, as this is just as harmful, if not more so, than a heavy tone arm. The vertical tracking force should be enough to hold the needle in the groove while it is being forced back and forth. If the horizontal tracking force overcomes the vertical tracking force, the needle will bounce in the groove and cause damage. It must be held firmly in the groove. If your phonograph has a recommended vertical tracking force range, use this range. Hopefully it is below 14 grammes. My dad's Dual phonograph (made in Germany) is capable of tracking forces between 2 and 3 grammes. The lateral tracking force is, of course, extremely light, which allows for an extremely light vertical pressure. To do so with my blunderbus steel needle phonograph would cause the needle to gouge out the sides of the grooves and skate across the record. Thankfully I have changed the cartridge on that phonograph to a much gentler one (the Zenith Cobra).

Thomas