Epidiorite

Epidiorite is a dark rock studded with garnet crystals. It is formed when dolerite dykes are subjected to great heat and pressure as in an orogeny such as the Caledonian Orogeny. There are interesting outcrops beneath the bridge at a Killiecrankie. If you descend to the river from the bridge you will see dark wavey bands of epidiorite near the River Garry which cross over the river bed and reappear on the other side.

Garnets are semiprecious but the ones at Killiecrankie are rather soft. Normally garnet is hard and it is 6 on Moh's scale of hardness. If is used as an abrasive (garnet paper) as well as in jewellery.