Diamond testers

I was given a diamond tester recently but i can not figure out how it
works. im hopeing someone can help.the instruments manufacturer’s
name is DAVENPORT & WATERHOUSE instruments ltd. Auckland, New
Zealand. it is housed in a metal box 4"wide 5"high 3"deep the
instrument display has a needal arm that goes from START TEST to
FINISH TEST on the lower midle of the box is a black butten that
clicks in when pushed with a red light directly below. to the right
is a nob when turned will move the needle. it plugs into 110 or 220V
/ 50/60Hz / 10VA. model U B O 3. s/n 1209 it has a dipro diamond testing probe.