Ribosian crown jewels

The Ribosian crown jewels were ceremonial relics of Ribos, greatly revered by the Ribosians. (TV: The Ribos Operation)

Protection
They were stored in the relic-room of the castle of Shur along with ceremonial regalia and sacred relics, guarded by the Shrieves, led by a captain. A curfew was in place at night in Shur, at which time the doors to the relic-room were locked and barred, and a Shrivenzale was let into the room. The case they were stored in was protected by multi-levered interlocks, and the penalties for trying to steal from the relic-room were enough to discourage the Ribosians from any attempt to break in. (TV: The Ribos Operation)

However, the Shrieves were unprofessional, and Garron and Unstoffe managed to break in through the Shrivenzale feeding hatch and plant what appeared to be a piece of jethrik among the crown jewels to try to make the Graff Vynda-K believe it was plentiful on Ribos in order to trick him into buying the planet from them. The Doctor also broke in on a search for a segment of the Key to Time (disguised as Garron's piece of jethrik), sneaking past the sleeping guard and unlocking the door with his sonic screwdriver. (TV: The Ribos Operation)

Additional information

 * Romana I was impressed by the crown jewels, describing them as "magnificent jewellery". (TV: The Ribos Operation)
 * Garron pointed out their workmanship, claiming they were all made by hand with simple implements. Trying to impress Graff Vynda-K, he described them saying: "There's a certain honest peasant artistry about these pieces that speaks to the soil, don't you think?" (TV: The Ribos Operation)
 * The Doctor speculated that the crown jewels were taken out of the relic-room perhaps twice a century. (TV: The Ribos Operation)