Thank you, KevH, for your information.
Before posting, I searched the GMB for all references to Ysart stones/caithness jewelry and read your posts as well as any others. A visit to the Ysart Glass site was also helpful. Unfortunately, I've done so much looking that now it's hard to recall what I saw where.
What I do not recall seeing or reading about is any reference to a number such as the "1" on the back of my brooch. Do you have any thoughts regarding this number and its meaning?